More than you want to know about dirt bikes,jetting,fuel, etc

[Jeff]

Heres all I could find on 2 stroke dirt bikes. i hope this gives you some
good bathroom material...

Jeff

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Intro
Carburetor tuning has the greatest effect on engine performance. When a
motorcycle manufacturer builds a bike, they usually install jets in the carb
that are too rich. The manufacturers sell the same model worldwide, so they
couldn't afford to install different jets in the carb to suit all the
different climates and types of fuel. In addition to the climate and fuel,
the manufacturer would also have to consider many other factors, such as the
terrain and type of riding. And then there is the most important jetting
consideration, the rider.

When I worked as a mechanic, I was in charge of jetting the bike over the
course of the day. During morning practice sessions, the track was usually
muddy and the air temperature was at its lowest point. I had to jet the bike
rich for practice because the air density was greater and the mud put more
of a load on the engine. Then I had to watch the rider and the bike perform
on different sections of the track. I would go to the obstacle on the track
that presented the greatest load on the bike, typically an uphill straight
section. I'd listen to my engine and watch the rider. I'd listen for pinging
or knocking noises or excessive smoke from the pipe. I would watch to see if
the rider had to fan the clutch a lot and how my bike pulled in comparison
to others. Getting feedback from the rider is difficult because they are
concentrating on riding not the bike's performance. At a pro national there
is one practice session, followed by a series of qualifiers and eventually
two race motos. The time spacing of the riding sessions over the course of
the day was such that I had to compensate the jetting two or three times.
Otherwise, the bike would either seize from being too lean in the morning or
run too rich for the second moto.

Race mechanics have different techniques for carb jetting. These techniques
range from asking other mechanics what jets they are running to using
precise measuring gauges to monitor the engine performance. In motocross
races, where most of the riders are of equal skill levels, a holeshot in the
start can mean the difference between a place on the podium and 30 minutes
of roost in your face! The difference in horsepower between the bike that
gets the holeshot and the bike that brings up the back of the pack may only
be a few ponies! The race mechanic can give his rider an awesome advantage
if he carefully monitors the carb jetting.

This section will give you insight into the carb tuning process, from
diagnosing mechanical problems that mimic poor jetting to tuning tools such
as gauges. It will also give you tips on a jetting method that I've
developed called the "ride-and-feel" method," which I consider to be the
best method It's a technique that I teach to all the riders I've worked
with. You don't need any fancy tools, just the ability to make observations
while you ride.

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The Differences in Two-Stroke and Four-Stroke Carbs
The difference between a two-stroke and four-stroke engine is intake
velocity. Two-stroke engines have lower velocity so the needle jet has a
half-moon shaped hood protruding into the venturi to produce a low pressure
area that aids in drawing the fuel up through the needle jet. Four-stroke
carbs need to atomize the fuel more so than a two-stroke carb because so
much of the fuel shears along the intake port and separates from the mixture
stream. Four-stroke carbs have more jets and finer adjustment screws, plus
they usually are equipped with an accelerator pump. A typical state of the
art four-stroke carb is the Kehin CR.

The latest trend in two-stroke carbs features a pump that sprays fuel into
the venturi from 1/4th to 3/4th throttles. In the past, carb manufacturers
made jet needles that attempted to compensate for the natural lean condition
of the mid-range but that compromised the jetting at full throttle. The
auxiliary pumps are powered by electricity supplied by the alternator (about
5 watts) and controlled by either a throttle position or an rpm sensor.

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Identification Guide to Popular Carb Types
On two-stroke engines, several different model carbs have been used over the
years, but there are basically two big carb manufacturers. Kehin and Mikuni
are two popular brands of Japanese carbs used on nearly every dirt bike.

Kehin has several different models. The most popular ones are the PJ, PWK,
and PWM. The PJ is used on Honda CR125, 250, and 500 models from 1985-1997
The slide is oval shaped and there are no additional pumps, its just a
simple carb. In fact it's so simple that the choke and idle screw share the
same jet. The PWK was the next step up from the PJ. The PWK has a crescent
shaped slide and a separate idle circuit from the choke. The PWK is used on
Kawasaki KX125, 250, and 500 models from 1990-97. The latest version of the
PWK features a pump to supply extra fuel in the mid-range. The PWM is
similar to the older PWK (no pump) and the overall length is shorter.

Mikuni has several different model carbs too. The original model VM had a
round slide. There are many different parts available including needle jets
of different diameters and jet needles with different taper angles and
diameters. The next model was the TMX, which became available in 1987. It
was a flat-slide carb, which offered a greater peak flow rate. The TMX was
revised several times, becoming smaller with fewer parts. The TMS carb
introduced in 1992 had no main or pilot jet. The slide and jet needle
handled all the jetting. That carb worked great on 250cc bikes but never
became popular. The PM is the latest Mikuni model. It features an oval
crescent shaped slide and a very short body. That carb comes standard on
Yamaha YZ125 and 250 1998 and newer models.

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Carburetor Parts and Function
A carburetor is a device that enables fuel to mix with air in a precise
ratio while being throttled over a wide range. Jets are calibrated orifices
that take the form of parts such as pilot/slow jets, pilot air screw,
throttle valve/slide, jet needle, needle jet/spray-bar, air jet, and main
jet. Fuel jets have matching air jets, and these jets are available in many
sizes to fine-tune the air-fuel mixture to the optimum ratio for a
two-stroke engine, which is 12.5: 1.

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Fuel Jets, Air Jets, and Throttle Positions
Three circuits control the air: the air-screw, the throttle slide, and the
air jet. Four circuits control the fuel: the pilot/slow jet, the
spray-bar/needle jet, the jet needle, and the main jet. The different air
and fuel circuits affect the carb jetting for the different throttle-opening
positions, as follows:

Closed to 1/8 throttleÑair screw and pilot/slow jet
1/8 to 1/4 throttleÑair-screw, pilot/slow jet, and throttle slide
1/4 to 1/2 throttleÑthrottle slide and jet needle
1/2 to full openÑjet needle, spray-bar/needle jet, main jet, and air jet
(Note: On many modern carbs the spray-bar/needle jet and air jets are
fixed-diameter passages in the carburetor body and cannot be altered.)

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Basic Carb Service
Nobody likes to fiddle with a carb if they don't have to. Wedged in between
the engine and frame with tubes, cables, and wires sprouting out
likespaghetti, carbs are a pain to work on. Carbs require cleaning just like
anything else, and some careful observations can save you big money in the
long run. Start by pressure washing the bike, especially around the bottom
of the carb where roost from the tires and oil from the chain accumulate.
Take care when removing the carb, it's easy to damage the cable. Its better
to remove the sub-frame so as to enable unrestricted access to the carb.
This will also make it easier to route the vent hoses in their proper
positions too. When you remove the carb look at the vent hoses. Are they
melted from heat or clogged with mud? If so that can cause a vapor-locking
problem in the float bowl and make the engine bog.

Remove the top of the carb and disconnect the cable from the slide. Is the
cable frayed or kinked? Is the rubber dust cover missing? If so then replace
the cable. Now remove the float bowl, jet baffle (white plastic shroud
around main jet), float and fuel inlet needle, and the air-screw. Shake the
floats and listen for fluid that may have seeped inside. If so replace the
floats otherwise the engine might suffer from constant fuel flooding. Check
the fuel inlet needle. It has a Viton rubber tip and occasionally fuel
additives and dirt damage the tip. Also check the spring-loaded plunger on
the opposite end of the tip. If the spring doesn't push the plunger all the
way out then replace it. Check the air-screw, there should be a spring and
o-ring on the end of the needle. The spring provides tension to keep the
air-screw from vibrating outward and the o-ring seals out dirt and water
from entering the pilot circuit. Next check the bell mouth of the carb. Look
for the two holes at the bottom of the bell mouth. The one in the center is
the air passage for the needle jet and the other hole offset from center is
the air passage for the pilot circuit. It's typical for those passages to
get clogged with dirt and air filter oil. That would cause the engine to run
rough because without a steady stream of air to mix with and atomize the
fuel, raw fuel droplets make the jetting seem rich.

Once the carb is basically stripped down (pilot/slow and main jet still in
place) you can flush the passages. Get an aerosol can of brake or carb
cleaner from an auto parts store. Make sure you get the type with the small
diameter plastic tube that attaches to the spray tip. Direct the tip into
the airscrew passage. When you spray the cleaner you should see it flow out
the pilot/slow jet and the air passage in the bell mouth. Next spray through
the pilot/slow jet, look for flow through a tiny passage located between the
venturi and the intake spigot. Spraying cleaner through these passages
insures that the low speed air and fuel circuits are open and free flowing.
The last area to flush with the carb cleaner is the slide bore and slide.
Dirt tends to trap there, causing the mating surfaces to develop scratches
that could cause the throttle to stick!

Just a small amount of water and dirt can get trapped in the tiny passages
of the carb and cause havoc with jetting or even engine damage. How often
should you service the carb? When it gets dirty! For example if you ride in
muddy wet conditions you should at least check the vent hose. If the riding
conditions are dusty and your air filter is covered with dirt, then it's a
good idea to do a basic carb servicing.

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Mechanical Problems
The process of jettingÑchanging air or fuel jets in order to fine-tune
engines' performanceÑis very simple. Jetting becomes complicated because
mechanical problems sometimes mimic improper jetting. This causes you to
waste time and money trying to correct the problem with expensive carburetor
jets.

Before you ever attempt to jet a carb, make sure the engine doesn't have any
of the problems in the following list. If you are in the process of jetting
a carb and you are stumped with a chronic problem, use this section as a
guide to enlightenment!

Crankcase air leaksÑAir leaks can occur at the cylinder base, reed valve, or
the magneto seal. Air leaks make the throttle response sluggish and may
produce a pinging sound. That sound occurs when the air-fuel mixture is too
lean.

Crankcase oil leaksÑThe right-side crankcase seal is submerged in the
transmission oil. When this seal becomes worn out, oil can leak into the
crankcase. The oil is transferred up to the combustion chamber and burned
with the air-fuel mixture. The oil causes the spark plug to carbon-foul.
This mechanical problem makes the jetting seem to be too rich.

Coolant-system leaksÑCoolant systems leaks commonly occur at the
cylinder-head gasket. When the coolant leaks into the combustion chamber, it
pollutes the air-fuel mixture and causes a misfire or popping sound at the
exhaust pipe. Check the engine's coolant level frequently. Hondas and
Kawasakis have characteristic coolant leaks because they use steel head
gaskets. Yamahas and Suzukis use O-rings to seal the head and cylinder.
Coolant-system leaks lower the engine's peak horsepower. It makes the engine
run as if the air-fuel mixture is too rich.

Carbon-seized exhaust valvesÑThe exhaust valves sometimes become
carbon-seized in the full-open position. This mechanical problem can make
the engine run flat at low rpm and make the slow-speed jetting seem lean.
The carbon can be removed from the exhaust valves with oven cleaner. Clean
the exhaust valves whenever you replace the piston and rings.

Blown silencerÑWhen the fiberglass packing material blows out of the
silencer, excess turbulence forms in the silencer and the turbulence causes
a restriction in the exhaust system. This restriction makes the engine run
flat at high rpm.

Broken reed-valve petalsÑThe petals of the reed-valve can crack or shatter
when the engine is revved too high. This mechanical problem makes the engine
difficult to start and can also have a loss of torque. Expert rider should
switch to carbon fiber reed petals because they resist breaking at high rpm.
Novice riders should use dual-stage fiberglass reeds (Aktive or Boyesen).
These types of reed petals provide an increase in torque.

Weak sparkÑWhen the ignition coils deteriorate, the engine performance will
become erratic. Normally, the engine will develop a high-rpm misfire
problem. Check the condition of the coils with a multimeter.

Clogged carburetor vent hosesÑWhen the carburetor vent hoses get clogged
with dirt or pinched closed, the jetting will seem to be too lean, so the
engine will run sluggish. Always check the condition of your carburetor vent
hoses. Make sure there is no mud in the hoses and that the hoses are not
pinched between the suspension linkage.

Carburetor float levelÑWhen the float level is too low, the jetting will
seem to be too lean, so the engine performance will be sluggish. When the
float level is too high, the jetting will seem to be too rich.

Worn carburetor fuel-inlet needleÑWhen the fuel-inlet needle wears out,
excess fuel enters the float bowl and travels up the slow jet and into the
engine. This makes the carb jetting seem to be too rich. Replace the
fuel-inlet needle and seat every two years.

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Jetting Shouldn't Be Scary!
Jetting is the process of making adjustments to the air and fuel jet sizes
in order to fine tune the carburation to suit the load demands on the engine
and make the power delivery consistent and optimum. Too much anxiety is
placed on jetting. Most people just want to call me on the phone and ask
what jets they should put in their carb. That's an impossible question
because that the big dirt bike magazines attempt to answer just to increase
readership. People get confused because they read jetting specs in a
magazine, put those jets in their bike and seize the engine. Any quoted
jetting in this book is just a baseline. Most magazines don't list
parameters for their jetting specs like; Brand new bike running with VP C-12
fuel with Silkolene oil mixed at 30:1 and a NGK 8 spark plug, ridden by a
really slow lard-ass editor twisting the throttle on a hard-packed track.
Some part numbers and jet sizes are given in the Tuning Tips section for
models that definitely need certain jets in order to get the bike near the
baseline. There is an old saying that says you can fish for a man and feed
him for a day or teach him to fish and enable him to feed himself for life.
Here is a quick lesson on how to jet your dirt bike.

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The Ride and Feel Method
The most basic method of determining correct carburetor jetting is "ride and
feel." This method requires you to determine if the carburetor tuning is too
rich or too lean by the sound and feel of the engine. The first step is to
mark the throttle body in 1/4-throttle increments, from closed to full open.
Then, this method requires that you ride the motorcycle on a flat, circular
course. To check the carb jetting for throttle positions up to 1/2 throttle,
ride the motorcycle in second or third gear. Roll on the throttle slowly
from 1/4 to 1/2 open. If the engine is slow to respond and bogs (engine
makes a booooowah sound) then the carb jetting is too lean. You can verify
lean jetting by engaging the carb's choke to the halfway position. This will
make the air-fuel mixture richer and the engine should respond better. If
the carb jetting is too rich, then the engine will make a crackling sound;
the exhaust smoke will be excessive and the engine will run as if the choke
is engaged. Careful engagement of the choke can help you determine if the
jetting is rich or lean. Another important tip is to just change the jets
one increment at a time, either rich or lean, until the engine runs better.
Most people are afraid to change a jet because they think that the engine
will be in danger of seizing. Believe me, one jet size won't make your
engine seize but it could be the difference between running bad and running
acceptable.

To check the jetting for throttle positions from 1/2 to full open, ride the
motorcycle in third and fourth gear. (You may need to increase the diameter
of the circular riding course for riding in the higher gears.) Check the
jetting in the same manner as listed above. The carb jets that affect the
jetting from 1/2 to full throttle are the jet-needle, main jet, power jet
(electronic carbs) and the air jet (on four-strokes).

If you want to take this technique out to the racetrack, you can test the
pilot/slow jet when accelerating out of tight hairpin turns, the needle clip
position on sweeper turns and short straits, and test the main jet on the
big uphill or long straits. Of course be careful if you try to use the choke
technique because you could lose control when riding one handed.

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Jetting for Riding Techniques
Certain types of riders require jetting to compliment their technique. For
example beginner minibike riders will need slightly richer jetting on the
pilot/slow jet and the needle clip position to mellow the powerband and make
it easier to ride. Conversely desert racers who hold the throttle wide open
for long periods of time need rich main jets to compensate for the high
load.

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The Weather Makes The Biggest Difference!
The weather can have a profound affect on the carb jetting because of the
changes in air density. When the air density increases, you will need to
richen the air-fuel mixture to compensate. When the air density decreases,
you will need lean-out the air-fuel mixture leaner to compensate. Use the
following as a guide to correcting your jetting when the weather changes:

Air temperatureÑWhen the air temperature increases, the air density becomes
lower. This will make the air-fuel mixture richer. You must select jet sizes
with a lower number to compensate for the lower air density. When the
barometric pressure decreases, the opposite effect occurs.

HumidityÑWhen the percentage of humidity in the air increases, the engine
draws in a lower percentage of oxygen during each revolution because the
water molecules (humidity) take the place of oxygen molecules in a given
volume of air. High humidity will make the air-fuel mixture richer, so you
should change to smaller jets.

AltitudeÑIn general, the higher the altitude the lower the air density. When
riding at racetracks that are at high altitude, you should change to smaller
jets and increase the engine's compression ratio to compensate for the lower
air density.

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Track Conditions and Load
The conditions of the terrain and the soil have a great affect on jetting
because of the load on the engine. Obstacles like big hills, sand, and mud
place a greater load on the engine that requires more fuel and typically
richer jetting. In motocross, track conditions tend to change over the
course of the day. Typically in the morning the air temperature is cooler
and the soil wetter requiring richer jetting. In the afternoon when the
temperature rises and the track dries out, leaner jetting is needed in order
to keep the engine running at peak performance. Other changes for mud and
sand riding might include changing to a lower final-drive ratio (rear
sprocket with more teeth) to reduce the load on the engine and help prevent
it from overheating. Advancing the ignition timing will make the engine more
responsive at low to middle rpm.

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Fuel and Oil Mixture Ratios
When we talk about the "fuel" in the air-fuel mixture for a two-stroke
engine, we are really talking about a mixture of fuel and oil. If you richen
the pre-mix ratio (20:1 as opposed to 30:1) there is more oil and less fuel
in the same volume of liquid, which effectively leans the air-fuel ratio.
And this fact gives the clever tuner one more tool to use when the correct
jet is not available or when none of the standard jets are exactly right.
You can richen the jetting by slightly reducing the pre-mix ratio (less
oil). You can lean the jetting by increasing the pre-mix ratio (more oil).
The best part is that changes in the pre-mix ratio affect the jetting over
the entire throttle-opening range, but the changes in ratio must be small to
prevent excess wear from lack of lubricating oil or fouled plugs from too
much oil.

Pre-mix oils are formulated for a fairly narrow range of pre-mix ratios. You
should examine the oil bottle for the oil manufacturer's suggestion on the
pre-mix ratio. All production two-stroke dirt bikes have a sticker on the
rear fender suggesting that you set the pre-mix ratio to 20:1 That sticker
is put there for legal purposes. Always refer to the oil manufacturer's
suggestion on pre-mix ratios. In general, small-displacement engines require
a richer pre-mix ratio than do large-displacement engines because smaller
engines have a higher peak rpm than larger engines. The higher the engine
revs, the more lubrication it requires.

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Tuning Gauges
There are three types of gauges that professional tuners use to aid carb
jetting:
1. Relative-air-density (RAD) gauge
2. Air-fuel (AF) ratio meter
3. Exhaust-gas-temperature (EGT) gauge

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The following is a description of how each gauge functions and their
advantages.
RAD gaugeÑThis is the best gauge for dirt bikes because of the convenience.
The gauge is no good unless you get the jetting perfect once. The RAD gauge
provides you with an indication of how much the air density changes, helping
you compensate for the affects of changes in the air temperature, altitude,
and barometric pressure. The gauge is calibrated in percentage points. Once
you set the jetting with the ride and feel method, you can set the
calibration screw on the gauge so the needle is pointing to 100 percent.
When the air density changes, the RAD gauge will show the relative percent
of change. Using a calculator you can multiply the percentage change shown
on the RAD gauge by the jet size and determine the corrected jet size for
the air density. The pilot/slow and main jet have number sizes that
correlate with the RAD gauge, but the needle clip position can only be
estimated. Normally for every two main jet increments, the needle clip must
be adjusted one notch.

AF ratio meterÑThe AF meter measures the percentage of oxygen in the exhaust
gasses, and displays the approximate air-fuel ratio of the carb. The gauge
displays AF ratios from 10-16:1 The optimum AF ratio for a two-stroke engine
is 12:1. The AF gauge utilizes a lambda sensor that is inserted into the
center of the exhaust stream, approximately six inches from the piston in
the header pipe of a four-stroke and in the baffle cone of a two-stroke
engine. A permanent female pipe fitting (1/4in.) must be welded to the side
of the exhaust pipe in order to fasten the sensor. The weld-on fitting
set-up is also used on the temperature gauges, and the fitting can be
plugged with a 1/4in. male pipe fitting when the gauge is not in use. This
gauge is ideal for four-stroke engines.

EGT gaugeÑThe EGT gauge measures the temperature of the gasses in the
exhaust pipe by means of a temperature probe fastened into the exhaust pipe,
six inches from the piston. This type of gauge enables you to tune the carb
jetting and the pipe together, taking advantage of the fact that exhaust
pipes are designed with a precise temperature in mind.

An exhaust pipe is designed to return a compression wave to the combustion
chamber just before the exhaust port closes. Most pipes are designed for a
peak temperature of 1,200 degrees Fahrenheit. Most dirt bikes are jetted too
rich, which prevents the exhaust gasses from reaching their design
temperature, so power output suffers. Sometimes just leaning the main jet
and the needle-clip position makes a dramatic difference.

Digitron is the most popular brand of EGT gauge. It measures both EGT and
rpm. This gauge is designed for go-kart racing so its not suited for wet
weather conditions. It is designed to mount on the handlebars. That way the
rider can focus in on it. Once you have performed the baseline jetting, send
the rider out on the bike with the EGT. The rider observes the EGT to give
you feedback on the necessary jetting changes. Once the jetting is dialed,
we use the tachometer to check the peak rpm of the engine on the longest
straight of the racetrack. For example, if the peak rpm exceeds the point of
the engine's power-peak rpm, then change the rear sprocket to a higher
final-drive ratio (rear sprocket with fewer teeth) until the rpm drops into
the target range. An EGT gauge is ideal for dirt track bikes and go-karts,
where peak rpm temperature is critical.

READING SPARK PLUGS

Q: Is there any place on the Internet where I can see pictures of spark
plugs and learn how to read them?

A: I don't know of such a guide so I put one together for you, of the five
popular spark plug patterns for two-stroke engines.

Heavy Carbon

This plug has heavy carbon build up. This engine had a blown crank seal on
the tranny side of the crank. Tranny oil entered the crankcase and was
burned in the combustion chamber. Engines like this will billow thick blue
smoke out the exhaust pipe.

Wet Fouled

This plug is wet fouled. The spark plug's heat range is either too cold or
the carb jetting is too rich.

Sand Glazed

This plug has a shiny appearance. The engine had a problem with air filter
sealing. Sand entered through the filter and into the engine. The high
combustion temperatures caused the sand to melt and form glass around the
spark plug.

Melted Aluminum

This plug has tiny globs of aluminum packed around the insulator. The engine
suffered a meltdown from ignition timing that was advanced too much. The
heat could not transfer from the spark plug fast enough and the center of
the piston melted causing the molten aluminum to collect on the plug.

Perfect Color

This is a perfect plug. The color is mocha brown so the carb jetting is
optimum. The first three threads are black signifying the plug's heat range
is matched to the application. There are relatively low deposits considering
that this engine was run on regular pump petrol.

Two-Stroke Top End Rebuilding

By Eric Gorr

Intro
Maintenance and Inspection
The Piston
Cylinder and Exhaust Valve Cleaning Tips
Top End Assembly Tips
Big Bore Kits
10 TIPS FOR REBUILDING A TWO-STROKE TOP END
Top End FAQs
Two-Stroke Exhaust Valves Tips and Tuning
Kawasaki KIPS
Suzuki ATEV
Honda HPP
Yamaha Powervalve
KTM

Intro

Top-end rebuilding is the most frequent and costly service routine on
two-stroke dirt bikes. Every year, dirt bike riders waste loads of money on
top-end parts that didn't need to be replaced, or make costly mistakes while
performing repairs. This section will give you the dos and don'ts to easy
top-end rebuilding, plus some tips that aren't printed in your factory
service manual.

Before You Start

Thoroughly wash your bike because dirt stuck to the underside of the top
frame tube could break loose when servicing and fall into the engine! Use a
stiff plastic brush and hot soapy water to clean off the grit and grime
around the base of the cylinder, on the carburetor and intake boot, and
especially underneath the top frame rail. Degreaser can be used on metal
surfaces, but take care not to leave it on rubber or gasket surfaces.

Tools

You'll need at least some 3/8-inch-drive metric sockets and box wrenches
(open-end wrenches will round off the edges on the cylinder or head nuts,
and shouldn't be used for top-end rebuilding), a needle-nose pliers for
removing circlips, and a gasket tool to scrape the old gaskets away. For
soft tools, get some shop towels, aerosol oven cleaner, a Scotch-Brite pad,
a locking agent such as Loctite, a gasket scraper, a brush, and a bucket of
soapy water. Regarding measuring tools, you'll need a compression tester, a
feeler gauge, and a digital vernier caliper.

Compression Testing

A compression tester is a useful diagnostic tool, and readily available from
Sears or auto parts stores. Buy the threaded type and make sure the kit
comes with an adapter that matches the spark plug threads of your engine.
Performing a compression test is simple. Start by removing the spark plug,
thread in the adapter, and hold the throttle wide open and the kill button
on. This will prevent any spark and enable the engine to draw in maximum
airflow. Then kick-start the engine several times until the needle on the
pressure gauge peaks. The pressure reading depends on two main factors; the
compression ratio and the altitude at which the engine is being tested. The
compression ratio will also depend on if the engine is equipped with exhaust
valves and their condition. When the exhaust valves are in the closed
position the compression ratio will be greater than if the valves are
carbon-seized in the open position. The difference may yield a pressure
reading 25 psi. The quality of compression testers varies greatly. The main
thing that a compression tester can identify is a change in condition.
Whenever you rebuild the top end, take a compression pressure reading and
mark it down. When the pressure changes 20% check the condition of the
piston and rings. Pistons usually last twice as long as rings.

Crankcase Pressure Testing

The crankcase of a two-stroke engine is sealed from the tranny. It's
important that the two crankshaft seals be in optimum condition. One side of
the crankshaft uses a dry seal and the other a wet seal. The dry seal runs
on the magneto side and the wet seal runs in oil on the tranny side. When
the dry seal wears, the crankcase sucks in hot air, causing the mixture to
run lean and overheat the engine. When the wet seal wears, the crankcase
sucks in tranny oil, causing the engine run rich and eventually wet-foul the
spark plug.

A crankcase pressure test involves the use of a vacuum pump with spark plug
adapter, and rubber plugs to block off the intake and exhaust manifolds of
the cylinder. The piston must be positioned at BDC to allow the transfer
ports to be wide open linking the bore and the crankcase. The hand-pump
produces vacuum pressure up to a standard setting of 5 psi. The normal
bleed-down pressure loss is 1 psi per minute. Cylinders with complicated
exhaust valve systems can be difficult to block-off air leaks, and harder to
test. Crankcase pressure testing kits are available from Motion Pro.

If I suspect that an engine has an air leak in the crankcases, I do a visual
test. Start by power washing the engine clean. Then remove the magneto
cover. Spray the magneto clean with an aerosol can of brake cleaner. Make
sure to use a non-chlorinated type of cleaner (green colored can). Now spray
baby powder to all the suspect areas of the engine. Spray the powder on the
crankcase around the magneto, at the crankcase seam line, the cylinder base,
and the reed valve. Run the engine for a while, the white baby powder will
highlight any fluid or air leaks on the engine. The baby powder test is much
better than the alternative test of blowing raw propane gas at different
areas of a running engine and listening for a change in the idle rpm. That
is dangerous because it involves flammable gas and a hot engine with random
electrical shorts.

Maintenance and Inspection

A thorough top-end rebuild requires removing the reed valve, cylinder head,
and cylinder. You should tear down your top end periodically and inspect the
reed valve, cylinder head, cylinder, piston, and so on. Use the following
chart to determine when you should tear down your bike:

Displacement: 80cc 125cc 250cc 500cc

Tear down after: 5 hours 10 hours 20 hours 40 hours

Note that air-cooled bikes should be inspected more frequently. Also, you
may want to inspect more often if you are riding in fine sand or lots of
mud. When you tear down the engine, inspect each system and look for the
following trouble signs.

Reed Valve

Check the reed petals for open gaps between the sealing surfaces. In time,
the reed petals lose their spring tension, and the back-flow can cause a
flat-spot in the throttle response. Stock nylon reeds tend to split at the
edges on bikes that are constantly over-revved. Expert riders find that
carbon fiber reeds last much longer.

Cylinder Head

Check the head at the edge of the chamber for erosion marks—a sign that the
head gasket was leaking. If the head or top edge of the cylinder is eroded,
it must be turned on a lathe to be resurfaced.

Cylinder

All cylinder bases use aligning (dowel. pins around two of the cylinder base
studs. These pins are made of steel, and after heavy power washing, they get
corroded. That makes it difficult to remove the cylinder from the
crankcases. Never use a pry bar! That will damage the cylinder. Instead use
a plastic mallet to hit upward on the sides of the cylinder at a 45-degree
angle. Alternate from left to right sides so the cylinder lifts up evenly.
After you remove the cylinder, stuff a shop towel into the open crankcases
to prevent debris from entering the engine.

The Different Types of Steel-Lined and Plated Cylinders

There are two types of cylinder bores used on dirt bikes, steel or cast iron
sleeves or ones with plating on the aluminum. Most dirt bikes made after
1989 have plated cylinders. You can check a cylinder with a magnet. If it
sticks to the bore then it is a sleeve. If it doesn't stick then it is
plated. There are three types of plated cylinders, Kawasaki Electrofusion,
hard-chrome, and nickel silicon carbide. There are several variations of the
nickel silicon carbide process but the most common trade name is Nikasil.
The nickel-based processes have many advantages over hard-chrome,
Electrofusion, and sleeving. Nickel attracts oil and is an excellent carrier
material for silicon carbide particles, a wear resistant material that
carries the load of the piston. This material is electro-plated right on to
the aluminum cylinder for the optimum thermal efficiency. Nickel can be
honed with diamond stones which leave distinctive peaks and valley scratches
in the cylinder wall which retain oil and provide a certain bearing ratio
between the running surfaces of the bore. It's possible to rebuild a plated
cylinder by fitting it with a sleeve. However you can expect to pay more for
bore maintenance over the life of the bike, and lose thermal efficiency and
horsepower. Plated cylinders are harder and last longer than sleeved
cylinders. Kawasaki cylinders with the original Electrofusion coating or
hard-chromed cylinders can be repaired with nickel plating or sleeving.
Steel or cast iron sleeves cannot be nickel plated unless they are separated
from the aluminum cylinder. The reason is that the pretreatment for the
plating would disintegrate the aluminum. There are four companies that
replate cylinders in the USA. The average price to replate a cylinder is
about $200.

The Piston

Some unfortunate guys do more damage replacing the piston than the actual
wear on the piston! Remove the circlips with a small needle-nose pliers and
throw them away. It is a common mistake to reuse circlips, but the cheap
spring-steel wire clips will fatigue and break if you install them for a
second time.

After removing the circlips, you have to remove the piston pin. Never use a
hammer and punch to remove the pin. That will damage the connecting rod and
needle bearings. Instead, use one of the pin-extractor tools available from
your local franchised motorcycle shop. You can also grasp the piston with
one hand and use a 3/8-inch socket extension to push the pin out with your
other hand.

Too many people replace their pistons too often. The exact service interval
for your bike depends on how hard the bike was run, for how many hours, the
quality of the lubrication, and the amount of dirt or other debris in the
intake air. Bikes that are run hard with dirty air filters may wear out
pistons in only 6 hours, while bikes that are ridden easy with clean filters
and adequate fuel octane may last 60 hours.

Measuring the Piston

The best thing to do is measure the piston with a caliper. Digital calipers
cost about $100 at industrial tool companies such as Enco or Harbor Freight.
A digital caliper is easy to use and gives accurate measurements on the
piston diameter and cylinder bore. Measure the widths of the piston (front
to back) just above the intake cutaway because this is the widest point of
the piston. Check the maximum wear specs in your service manual. Check the
piston for detonation marks in the crown, cracks in the skirt, or seizure
marks. Look at the underside of the piston crown for a large black spot. The
spot is burnt oil deposits that adhered to the piston because the piston
crown temperature was too hot. This is an indication that the carb's main
jet needs to be richer.

Letter Designations on Cylinders and Pistons

The Japanese manufacturers use a letter designation system for plated
cylinders. They intend for you to order replacement pistons based on the
letter designation printed or stamped on the cylinder. This is the reason
why they need this type of system. In mass production you can't guaranty
that all parts will be exactly the same size. The size variance is based on
an acceptable level of quality. Tool bits become dull, temperatures of
machine tools change through production runs, and machine operators have
inconsistent performance. The Japanese manufacturers have between two to
four different sized pistons and cylinders. Normally labeled A, B, C, and D.
If they only had one size, the piston to cylinder wall clearance would vary
between .001 to .006 inches. In the standard Japanese alpha labeling system,
A denotes the smallest bore or piston size and every letter after that is
slightly larger, usually in increments of .0015 inches. The danger is that
if you try to put a D piston in an A cylinder the piston to cylinder wall
clearance will be so tight that a seizure might occur.

Pro-X Oversize Piston Kits

Pro-X is a marketing company that sells the surplus pistons from the
Japanese company ART, which makes all the cast pistons for the Japanese
motorcycle manufacturers. These pistons are the same quality as the OEM
pistons, and they are available in sizes larger than the alpha pistons
available from franchised dealers. Also the Pro-X pistons are usually priced
lower than the OEM pistons. If the cylinder bore is slightly worn (up to
.005 inches) with only a small area of bare aluminum exposed, you can
install a Pro-X oversize piston. The Pro-X pistons are graded oversize in
smaller increments than Wiseco pistons, but a wider range than the OEM
pistons. For example, Wiseco sizes are .010 inches and Pro-X is .001 inches
increments. Before attempting to order a Pro-X piston, you must measure the
cylinders bore at the smallest point and allow .002 inches clearance between
the piston and cylinder.

Measuring the Ring Gap

The best way to know if the rings are worn is to measure the ring end gap.
Put the ring in the cylinder and use the piston to push it down about 1/2
inch from the top evenly spaced. Now use a feeler gauge to measure the width
of the ring gap. Normally, the maximum gap is 0.018–0.025 inch.

Cylinder and Exhaust Valve Cleaning

Does your cylinder have burnt-on mud on the outside, heavy brown oil glazing
on the cylinder bore, or gooey oil on the exhaust valves? If so, here is a
tip for cleaning those parts without flammable cleaners. Go to the grocery
store and get a can of aerosol oven cleaner. This stuff is great for
cleaning the carbon from the exhaust valves without completely disassembling
them. CAUTION: Oven cleaner attacks aluminum, so don't leave it on the
cylinder for more than 20 minutes. Oven cleaner can be used on both steel
and plated bores.

The oven cleaner will help loosen the oil glazing on the cylinder walls.
Then, you can use a Scotch-Brite pad to hone the cylinder walls in a
crisscross pattern. Wear rubber gloves when you use oven cleaner and flush
the cylinder afterwards with soapy water. This will neutralize the acid in
the oven cleaner and break the molecular bond of the oil, so the debris can
be rinsed away. Sleeved (especially Kawasaki cylinder bores) are vulnerable
to corrosion after cleaning. Spray some penetrating oil on the cylinder bore
to prevent it from rusting.

Caution: Certain types of cylinders corrode quickly after the cleaning
process, so spray the bore area with penetrating oil to displace the water.

Honing the Cylinder Bore

Many people have emailed me with questions regarding honing cylinder bores.
If you want to buy a hone to deglaze bores or polish off small scratches,
then a ball-hone is the best choice. Ball hones are manufactured by Brush
Research in Los Angeles, under the brand name Flex-Hone. These hones are
available under different labels and they are most easily available from
auto parts stores. Buy a size that is 10% smaller than the actual bore size.
These hones are available in several different materials and grits but the
profile that bests suits both steel and plated cylinders is aluminum oxide
240 grit. A ball hone cannot remove material from the cylinder bore,
especially on the hard nickel plated bores. However a ball hone can polish
down the peaks of the original hone scratches and increase the bearing
ratio. In other words the piston will be touching a greater percentage of
the bore. Sometimes that makes the piston wear quicker but if you have to
ball hone the bore to remove scratches, it's a compromise. The one type of
hone that you should never use on a two-stroke cylinder is a spring-loaded
finger hone. The sharp edges of the stone will snag the port edges and most
likely damage the hone and the cylinder.

Top End Assembly

1. Install one of the circlips in the piston with the opening facing away in
the 6 or 12 o'clock position.

2. Grease the cylinder-base alignment pins.

3. Set the exhaust valves in the closed position.

4. On cylinders with reed valves, leave the intake port open because you
will need to reach in through the port to push the piston-ring ends back in
place.

5. The best way to slip the piston into the bottom of the cylinder is to
rotate the rings toward one side of the locating pins and squeeze the rings
with your middle finger and thumb. That will leave your other hand free to
position the cylinder.

6. There are two methods used to assemble to top end. The first method is to
attach the piston to the connecting rod and lower the cylinder on to the
piston assembly. The second method is to install the piston assembly into
the cylinder and lower the cylinder and piston on to the connecting rod. The
second method is easier but involves pinning the piston and installing one
circlip with a minimum amount of free space.

7. Take care to align the exhaust valve control mechanism as the cylinder is
bolted to the crankcases.

Gasket Hygiene

The oven cleaner you used to clean the cylinders will help loosen the old
gasket material so you can remove it. Carefully scrape the gasket off with a
gasket scraper. Never use a flat screwdriver to remove the old gaskets
because the aluminum surfaces of the head, cylinder, and crankcases are
easily gouged. If these surfaces are gouged on your engine, they should be
draw-filed flat to prevent air or coolant leaks.

Never reuse paper gaskets; always replace them with new gaskets, and spray
sealer on the paper gaskets, so they will seal better and will be easier to
remove the next time. The new-style steel gaskets can be cleaned and reused
a few times, but you'll need to spray the gasket with a sealer such as
Permatex Spray-A-Gasket or copper-coat.

Keep a Logbook

Keep a logbook that tracks the number of riding days and the periodic
maintenance. From reviewing the log, you will learn how often you need to
service the top end if you record the measurements of the ring gap and the
piston diameter. A logbook also gives you greater leverage when you try to
sell your used bike for a premium price.

Big Bore Kits

One of the best ways to increase horsepower is to increase displacement by
overboring the cylinder. This can be ideal for play or Vet Class riders,
where the increased displacement won't be illegal for your race class. When
done right, a big bore kit can give you more power everywhere rather than an
increase in only the top or the bottom of the powerband. Such increases are
typically more usable and give you more power where you need it.

Piston manufacturers such as Wiseco make oversize piston kits for popular
model bikes. These kits boost the displacement of the cylinder to the limit
of a racing class or to a larger displacement class, for example: 80cc to
100cc, 125cc to 145cc, 250cc to 265cc or 300cc, and 495cc to 550cc.

The AMA has a limit of overboring any cylinder used in amateur modified
classes. The limit is 2 millimeters. Wiseco makes a line of Pro-Lite pistons
for this purpose. Normally no head modifications are needed, but cylinders
with exhaust valves that operate close to the cylinder bore will need to be
trimmed for clearance. Cylinders that use steel head gaskets will require
oversize gaskets. Cometic makes 2 millimeter oversize and big bore gasket
kits. The process of overboring and electro-plating a cylinder can be a cost
effective way to save a cylinder that suffered a top end failure and scored
the cylinder wall.

Riders competing in the AMA veteran class can ride a bike with any
displacement. Riders competing in hare scrambles and enduro can race the
200cc class with a 125 converted to any displacement. AMA motocross and
enduro racers can make the 250cc bikes legal for open class by increasing
the displacement a minimum of 15 percent (to 286cc). Wiseco makes
74-millimeter piston kits to convert the popular 250s to 300cc. Be careful
if you decide to go with a big bore kit, though. If the overbore is not
performed properly, though, it can result in the wrong kind of power or, at
worst, a ruined cylinder. When you change the displacement of the cylinder,
there are so many factors to consider, such as port time-area, compression
ratio, exhaust valves, carb jetting, silencer, and ignition timing. Here is
an explanation of what you need to do when planning to overbore a cylinder.

Also, you should at least consult with an expert before tackling a big bore
kit. To get the most from an overbored engine, you need to make sure the
carburetion, exhaust, porting, and timing are all adjusted to suit the
larger bore.

Port-Time Area

The term port-time area refers to the size and flow range of the intake and
exhaust ports, relative to rpm. The ports enter the cylinder bore at angles.
When the cylinder is over-bored the transfer ports become lower and wider.
The same thing happens to the exhaust port. This effectively retards the
port timing and reduces the total degrees of duration. When the displacement
of the engine increases, so does the demand for more port-time-area.

If you just overbored and plated a cylinder, it would have much more low-end
power than stock but the top-end power would suffer. Normally tuners have to
adjust the ports to suit the demands of the larger engine displacement. The
proper dimensions for the ports can be calculated using a computer program
from Two-Stroke Racing (TSR) www.tsrsoftware.com The program "PORTTIME"
enables tuners with limited math skills to run strings of formulas for
determining the optimum dimensions of the ports. Generally speaking, if the
ports in the overbored cylinder were raised to the same heights as the stock
cylinder, that would make the port timing sufficient to run with stock or
aftermarket exhaust systems.

Cylinder Head

After overboring the cylinder, the head's dimensions must be changed to suit
the larger piston. First, the head's bore must be enlarged to the finished
bore size. Then, the squish-band deck height must be set to the proper
installed squish clearance. The larger bore size will increase the squish
turbulence, so the head's squish band may have to be narrowed. The volume of
the head must be increased to suit the change in cylinder displacement.
Otherwise, the engine will run flat at high rpm or ping in the midrange from
detonation.

Exhaust Valves

When the bore size is increased, the exhaust valve-to-piston clearance must
be checked and adjusted. This pertains to the types of exhaust valves that
operate within close proximity of the piston. If the exhaust valves aren't
modified, the piston could strike the valves and cause serious engine
damage. The normal clearance between the exhaust valves and the piston
should be at least .030 inches or .75 millimeters

Carburetor

The larger the ratio between the piston's diameter and the carb's size, the
higher the intake velocity. Overbored cylinders produce higher intake
velocity which draws more fuel through the carb. Of course a larger engine
will need more fuel. Normally when you overbore an engine 15-20%, the slow
jet will need to be richened and the main jet will need to be leaned. Start
with the stock jetting and make adjustments after you ride the bike.

Ignition Timing

The ignition timing has a minimal affect on the poweband. Retarding the
timing has the affect of reducing the hit of the powerband in the midrange
and extending the top end over rev. "Overrev" is a slang term that describes
the useable length of the powerband at high rpm.

The scientific reason for the shift of the powerband to extremely high rpm,
is because the temperature in the pipe increases with the retarded timing,
and that enables the pipe's tuned length to be more synchronous with the
piston speed and port timing of the cylinder.

Advancing the timing has the affect of increasing the midrange hit of the
powerband, but makes the power flatten out at high rpm. The reason is that
the relatively long spark lead time enables for a greater pressure rise in
the cylinder before the piston reaches TDC. This produces more torque in the
midrange but the high pressure contributes to pumping losses at extremly
high rpm.

Pipe and Silencer

Because only the bore size is changed, you won't need a longer pipe—only one
with a larger center section. FMF's line of Fatty pipes work great on
engines that have been overbored.

Head Gasket

The head gasket will need to have the bore diameter increased to the
dimension of the new piston. If the head gasket overlaps into the cylinder
bore more than one millimeters on each side, it could contact the piston or
be susceptible to pressure blowouts.

10 TIPS FOR REBUILDING A TWO-STROKE TOP END

1) Before you disassemble your engine, power-wash the engine and the rest of
the vehicle. That will reduce the risk of dirt and debris falling into the
engine. Once you remove the cylinder, stuff a clean rag down into the
crankcases.

2) The cylinder and head use alignment pins to hold them straight in
position from the crankcases on up. The pins make it difficult to remove the
cylinder from the cases and the head from the cylinder. Sometimes the steel
alignment pins corrode into the aluminum engine components. Try spraying
penetrating-oil down the mounting studs before attempting to remove the
cylinder and head. Never use a flat-blade screwdriver, chisel, or metal
hammer to remove the cylinder. Instead use this technique; buy a lead-shot
plastic mallet, swing it at a 45-degree angle upwards against the sides of
the cylinder. Alternate from left to right, hitting the sides of the
cylinder to separate it from the cases evenly. Clean the steel alignment
pins with steel wool and penetrating-oil. Examine the pins closely. If they
are deformed in shape, they won't allow the engine parts to bolt together
tightly. This can cause a dangerous air leak or a coolant leak. The pins are
cheap at about $2 each. Replace them if they're rusty or deformed.

3) Never re-use old gaskets. Remove them with a razor blade or gasket
scraper. Don't use a drill-driven steel wool type pad to remove old gaskets
because they can remove aluminum from the cylinder and head. That will cause
a gasket to leak.

4) Always check the ring end gap on a new ring by placing it in the cylinder
between the head gasket surface and the exhaust port. The gap should be
between .012 to .024 inches.

5) Always install the circlips with the opening facing straight up or down,
that way inertia will hold it tight into the clip groove. Place one clip in
the groove before installing the piston on the connecting rod. Its easier to
install a clip with the piston in your hand rather than on the rod. There
also less chance that you'll drop the circlip in the crankcases.

5) Always install the rings on the piston with the markings facing up. Coat
the rings with pre-mix oil so they can slide in the groove when trying to
install the piston in the cylinder.

6) Always install the piston on the connecting rod with the arrow on the
piston crown facing towards the exhaust port.

7) The traditional way to assemble the top end is to install the piston
assembly on the connecting rod, compress the rings, and slide the cylinder
over the piston. That can be difficult with larger bore cylinders, or if
you're working by yourself. Try this method instead. Install one circlip in
the piston, install the piston into the cylinder with the pin hole exposed,
install the piston pin through one side of the piston, position the cylinder
over the connecting rod and push the piston pin through until it bottoms
against the circlip, install the other circlip. It only takes two hands to
install the top end using this manor and there is less chance that you'll
damage the rings by twisting the cylinder upon installation.

8) On cylinders with reed valves and large oval intake ports, take care when
installing the piston assembly in the cylinder because the rings are likely
to squeeze out of the ring grooves. Use a flat-blade screwdriver to gently
push the rings back in the grooves so the piston assembly can pass by the
intake port.

9) For steel head gaskets, place the round side of the "bump" facing up.
Don't use liquid gasket sealer; use aerosol spray adhesive types instead.
For hybrid fiber/steel ring head gaskets, place the wide side of the steel
rings facing down.

10) When you initially start the engine after a rebuild, manipulate the
choke to keep the engine rpm relatively low. Once the engine is warm enough
to take it off choke, drive the vehicle around on flat hard ground. Keep it
under 2/3 throttle for the first 30 minutes. Two common myths for proper
engine break-in are; 1) Set the engine at a fast idle, stationary on a
stand. 2) Add extra pre-mix oil to the fuel. When the engine is on a stand
it doesn't have any air passing through the radiator and it is in danger of
running too hot. When you add extra oil to the fuel you are effectively
leaning the carb jetting. This can make the engine run hotter and seize.

Top End FAQs

Thin Sleeve Causing Seizures

Question: My 1987 CR125 has chronic piston seizure problems. The cylinder is
bored one millimeters oversize. The lower end was rebuilt so I know it
doesn't have a crankcase air leak. What could the problem be?

Answer: The original cylinder for your model bike had a very thin steel
sleeve. Honda only offers one oversize piston. When the sleeve is overbored
too far, the sleeve cannot transfer out heat into the water jacket
efficiently. The heat builds up over the exhaust port, and the piston melts.
You have two repair options: buy a new cylinder or install a new thicker
sleeve in the old cylinder. Wiseco offers thick sleeves and forged piston
kits.

Honda CR250 1988–91 HPP Problems

Question: My 1990 Honda CR250 is making me wacky. I tried to check the
exhaust valve system, and I don't think it works properly. I removed the
left-side valve cover from the cylinder, revved the engine and the valves
hardly moved. They don't open fully when the engine is revved, and they
don't close completely either. What is the most common cause of this problem
and how can I fix it myself?

Answer: The problem is that the HPP mechanism isn't fully engaged, and the
valves are just moving from the exhaust-gas pressure. The most common
problem with the 1988–91 CR250 HPP systems, is the improper engagement of
the governor control and the spindle rod that actuates the HPP valves. The
following procedure may cure the problem. Remove the top right valve cover
on the cylinder and the round-slotted access cover located under the water
pump on the right side engine cover. Insert an 8mm T-handle through the
access hole and onto the detent bolt that locks the governor control to the
cam spindle, and turn the bolt 1/4 turn counterclockwise. Now, the bolt has
disengaged the HPP system. Insert a straight-blade screwdriver into the slot
in the top of the right-side pinion shaft (from the top right side of
cylinder). Turn the pinion shaft counterclockwise 1/8 turn, and then turn
the detent bolt (located under the right-side engine cover) 1/4 turn
clockwise. It is important to release the spring tension from the pinion
shafts in the cylinder to engage the detent bolt. This procedure also
enables the HPP mechanism to be engaged without any chance of damage
occurring to the fragile cam spindle.

Top-End Big Bore

Question: I have an old cylinder for my 250. The bore was ruined when the
head gasket leaked, and there is severe erosion on the top edge of the
cylinder. I read your article on top-end rebuilding and had an idea and a
related question. I compete in amateur enduro events and the rules state
that the displacement of bikes competing in the open class must be a minimum
of 251cc. My question is, can I salvage this old junk cylinder by overboring
the cylinder to fit a Wiseco piston kit and have the bore re-plated? If yes,
will my bike be legal for the open class?

Answer: There are a number of companies offering cylinder repair services
and replating. The way to fix the erosion problem is to heli-arc weld
aluminum over the erosion and then re-face and bore the cylinder. WISECO and
L.A. Sleeve make oversize piston kits and gaskets for most Japanese dirt
bikes. The common overbore displacement sizes for 250s are 265, 285, and
310cc. After the cylinder is re-plated, the exhaust valves and the cylinder
head must be matched to the larger bore size. This involves special metal
machining and should be trusted to a qualified tuner or machinist. This type
of mod will enable you to race your 250 in the open class.

Kawasaki Air/Oil Leaks

Question: My son and I are just getting started in dirt-biking. Over the
winter I bought him a 1989 KX80 as a basket case. We are learning about dirt
bike repairs by rebuilding this bike. It's a lot like model building, only
the parts are old and greasy! We inspected the crankcases and noticed that
there was some oil leaking from the three oval-shaped plugs that are spaced
an equal distance around the main bearings. How can we repair this problem
without buying new crankcases?

Answer: Every Kawasaki dealer's service department has a Team Green book
with tips on how to repair common problems. Ask your dealer's service
manager for a copy of the Team Green bulletin. It has photos and drawings of
how to apply the epoxy over the crankcase plugs.

Top-End Seized After Rebuild

Question: I trail ride a 1989 YZ250. Last winter, I rebuilt the top end
after reading your article in Dirt Rider. The bore was so worn that I had to
skip to a one millimeter-oversize piston kit, just so the bore job would
clean up a severely worn spot below the intake port. After I rebuilt the top
end, I cycled the engine by letting it idle for three 15-minute sessions
with adequate cool-down periods in between. When I first rode the bike, I
heard some detonation noises but didn't think it was a serious problem,
until it seized. What could be wrong?

Answer: Your problem is simple. When a cylinder is overbored, the
displacement is increased and that boosts the compression ratio. Whenever a
cylinder is overbored more than 0.010 inches or 0.25mms the cylinder-head
diameter must be enlarged to the new bore size. Otherwise, the piston could
contact the head or the edge of the head surface that extends into the bore
could cause a hot-spot and pre-ignition. Also, the cylinder head's squish
band must be narrowed by enlarging the combustion-chamber bowl. This also
serves to increase the head's volume, thereby lowering the compression
ratio. This work must be performed on a lathe by a qualified tuner or
machinist. Average cost of this service is $50

Base Gasket Seeping

Question: I recently rebuilt the top end on my 1991 CR250. I was being as
careful as I could be while taking the cylinder off, but the dowels were
fused in pretty good and I had to pry it. Needless to say, I gouged the case
a bit. I smoothed it out with sandpaper and reassembled the engine. The bike
runs great, but a little oil seeps out of the cylinder-to-case mating
surface. I assume this is transmission oil? Would it be OK to use something
like a thin layer of Permatex Blue or Yamabond here? Would this make it even
more difficult to remove the cylinder in the future? Should I just let it
alone? The best price I could find on a new left side case was $215 and I'm
sure it would be a lot of work and a lot of replacing gaskets along the way.
Am I out of luck?

Answer: Air leaks can be very dangerous because the engine could rev
independent of the throttle. An inexpensive way to fix your bike's problem
is to draw-file the cylinder base and the crankcases. Then apply a thin
coating of Yamabond or any other brand of non-drying sealer to both sides of
the base gasket. The best technique for removing cylinders is to tap up on
each side of the cylinder with a lead-shot plastic mallet. Remember to put a
dab of grease on the cylinder-base dowel pins.

Frequency of Top-End Rebuilding?

Question: I have a 1990 RT180, and I don't think the rings or piston have
been replaced. I don't know if the top end has ever been rebuilt because I
bought the bike used. How long do piston and rings usually last on a
two-stroke engine like mine? How often should the piston and rings be
replaced, and should I replace them now?

Answer: Replace the piston and rings before they wear out. The time scale
varies between models, usage, and preventive maintenance. The only way to
determine the condition of your bike's top end is to disassemble the top end
and measure the piston diameter and the ring end gap. Compare the
measurement to the maximum wear specs published in the service manual.

Two-Stroke Exhaust Valves

Three words sum up exhaust valve maintenance: spoogey, gooey, and grungy. If
two-stroke exhaust valves didn't have such a dramatic effect on the engine's
powerband, I'm sure mechanics would remove them and beat them bits with a
hammer in frustration because there is little information given by the
manufacturers on how to diagnose and repair the exhaust valve systems on
well-used dirt bikes. This section is a guide to the characteristic
mechanical problems that occur to the exhaust valve systems of dirt bikes.
Plus we'll give you some tips on how to re-time exhaust valve systems.

How Exhaust Valves Work

An exhaust valve system is designed to increase the engine's low-end and
midrange power. There are three different designs of exhaust valve systems.
The first-generation design uses a variable-volume chamber mounted to the
head pipe to change the tuned length of the head pipe. A butterfly valve is
used to separate the surge chamber and the head pipe. At low rpm, the valve
is open to allow the pressure waves in the pipe to travel into the surge
chamber and effectively lengthen the pipe and reduce the pressure wave's
magnitude when it returns to the exhaust port. These systems were primitive
and not very effective on 125cc dirt bikes. Honda and Suzuki used this type
of exhaust valve system in the mid to late 1980s.

The second-generation design features valves that control the effective
stroke and the time-area of the exhaust port. These valves are fitted to the
sub-exhaust ports and the main exhaust port. The main exhaust-port valves
operate within close proximity to the piston to control the effective stroke
of the engine. The effective stroke is defined as the distance from TDC to
when the exhaust port opens. At low rpm, the engine needs a long effective
stroke, which results in a high compression ratio. At high rpm, the engine
needs a shorter effective stroke, longer exhaust duration, greater
time-area, and a lower compression ratio. Yamaha used this system starting
in 1982 on the YZ250. Honda's HPP system is similar and was used on the
1986–91 CR250 and 1990 to current-model CR125.

The third generation of exhaust valve systems attempts to change the
exhaust-port velocity, effective stroke, exhaust-gas temperature, and the
pressure of the compression wave. Yamaha and Suzuki started using these
systems on their 125s in 1995. Both companies employed a venting system to
the outside atmosphere. This is very complex because they are attempting to
affect the temperature and pressure of the returning compression wave to
synchronize it with the piston speed. The exhaust-gas velocity and the
effective stroke are controlled by two oval wedge valves that enter the
exhaust port at a 45-degree angle. The wedge valves partially block the
exhaust port, thereby boosting the gas velocity. Kawasaki's KIPS system uses
wedge valves in the main exhaust port to control the effective stroke, drum
valves in the sub-exhaust ports to control the time-area, and a surge
chamber to absorb the excess compression-wave pressure at low rpm.

The exhaust valves are opened and closed by a centrifugal governor
mechanism. The governor is mounted under the right side cover and is
gear-driven by the crankshaft. As the engine rpm increases, the governor
spins, thereby increasing the angular momentum of the four steel balls
encased in the governor. The steel balls fit into an angled ramp-and-cup
arrangement. A spring is used to provide tension on the steel balls. When
the momentum of the steel balls overcomes the spring's tension, and the
balls force their way up the angled ramp. A spool attached to the ramp,
enabling it to change its linear position with changes in rpm, and the spool
is attached to a linkage system that operates the exhaust valves in the
cylinder. Factory race teams have different combinations of springs, ramps,
and balls to tune the exhaust valve operation and enhance the powerband.

Exhaust Valve Tips and Tuning

Although exhaust valves use the same essential principles, the
implementation is different with each manufacturer. Also, each type has its
own flaws and fixes. The list below gives you tips on how to install and
service the most common exhaust valves, as well as some tuning tips

Honda HPP

Honda's HPP system started as a butterfly operated canister mounted between
the cylinder and pipe. It served to control the volume and length of the
exhaust pipe. It had little effect on the power and most aftermarket pipes
eliminated the canister. The butterfly was prone to carbon seizure and
required frequent maintenance. The next generation HPP was used on the
1986-91 CR250. This system featured two sliding valves that operated within
close proximity of the piston and effectively varied the exhaust port
time-area in accordance with rpm. The square valves moved horizontally
through a valve guide. The system was plagued with a mixture of design
problems and misinformation on how to service and re-time this complicated
exhaust valve arrangement. This section lists some common problems and some
tips for timing the system, installing the cylinder, and engaging the HPP
mechanism.

Common HPP Problems

Two main problems plague the HPP system: carbon fouling and
rack-and-cam-spindle damage. The square shape of the valves contributes to
the accumulation of carbon on one corner of the valve guide (stationary
part), in the corner of the guide that is directly in the exhaust gas stream
and this causes the valve to become carbon seized. Chamfering the
corresponding edge (one-millimeter) of the valve will eliminate this
problem. The rack and cam spindles are easily damaged when the cylinder is
installed incorrectly, or the HPP mechanism is engaged incorrectly. See the
photos for examples of damaged rack and cam spindle parts.

HPP Timing Procedure

Use the following procedure to time the HPP system:

1. Install the HPP valves and levers and tighten the pivot nuts. Place the
washer on the stud first, then the lever (marked left and right), and then
the flanged center bushing with the flange side facing up.

2. Turn the cylinder upside down. To position the rack correctly, slide it
to the left until it stops; then move it right 2mm. Rotate the rack so the
square notch faces you. Now the rack is in the correct position so you can
install the pinion shafts. Carefully turn the cylinder right side up without
changing the position of the rack.

3. Close the valves and install the left pinion shaft with the screwdriver
slot facing the one o'clock position. Install the right pinion shaft with
the screwdriver slot facing the eleven o'clock position (see photo for
correct positions). A simple way to determine if the pinions are mis-timed
to the rack is to look at the screwdriver slots. The wrong position is with
both slots facing twelve o'clock.

Installing the Cylinder and Engaging the HPP Drive

After timing the HPP mechanism, the cylinder is ready to be installed on the
crankcases. Here are some tips for installing the cylinder and engaging the
HPP drive mechanism:

1. Make sure the reed valve is removed from the cylinder. CR250s have such
large intake ports that the rings tend to slip out of the ring grooves
during installation of the cylinder. This takes the spring pressure off the
cam spindle. Now turn the engagement bolt 1/4 turn clockwise. You should
feel it positively lock into a groove and stop. Remember that the HPP
engagement bolt is a spring-loaded detent not a threaded bolt. Slide the
cylinder down onto the piston and rings, use a screwdriver to push the rings
back in the grooves until the rings clear the intake port.

2. The HPP mechanism should be engaged while the cylinder is being
installed, just to keep the cam spindle in position. The cylinder will stop
about 3mm from the crankcases because the cam spindle and the rack are
misaligned. Now disengage the HPP mechanism by turning the engage bolt 1/4
turn counter-clockwise. Grasp the right-side valve lever and wiggle it; the
cylinder should then drop evenly onto the crankcases.

3. Bolt the cylinder down tight. The best way to engage the HPP mechanism is
to insert a screwdriver in the right-side pinion shaft and turn it
counterclockwise. Now turn the engagement bolt clockwise. You should feel
the engagement bolt lock positively in position. If you try to rotate it too
far, you will bend the cam spindle and the system won't work at all, so
don't be a hammer-head! The best way to check the HPP system is to remove
the left-side valve cover from the cylinder, start the engine and warm it
up, then rev the engine. The valves should be fully closed at idle and fully
open when the engine is revved.

In 1992 Honda introduced the HPP system currently used on the CR250. This
system features a center valve for the main exhaust port and two rotating
drum valves to control the flow of the sub exhaust ports. This system also
features a return of the old resonator as used on the mid-eighties model.
The resonator improves the throttle response and mellows the powerband at
low rpm. A thin rod links the valves together and the whole system is mostly
self-scraping to prevent carbon build-up. The inside of the center valve has
an elongated passage where the tie rod travels. This elongated passage is
prone to carbon build-up over time (1-2 years). The carbon limits the range
of movement in the valves. The carbon is easily removed by using a small
diameter rat-tail file. The sides of the center valve and the drum valves
interface, and that area is prone to carbon build-up too. A wire brush or
file is an effective tool in cleaning the exhaust valves. Here is a simple
way to check the operation of this system. On the left side of the cylinder
there is a 17mm cap bolt that exposes a straight line mark in the left drum
valve. There is a corresponding mark on the cylinder. The "L" mark denotes
the low speed position of the valve and the "H" denotes the high-speed
position. To check the HPP, start the engine. At idle the valve should align
with the "L" mark. Then rev the engine, the valve should align with the "H"
mark. If the angle of the mark on the valve is slightly off, then the valve
probably needs to be de-carboned. This system is very easy to disassemble
and can only fit together one obvious way so we won't waste space on that
procedure. There is some aftermarket parts to adjust the performance of this
system for different types of dirt biking. Pro-Racing in England makes a
spacer for the right side valve cover. It serves to add volume and length to
the resonator part of the system. This is especially suited for enduro
riding where a smooth transition to the mid-range is important for better
traction. ESR (Eddie Sanders Racing) in California makes a replacement HPP
system that holds the valves wide-open. The center exhaust valve is thinner
which enables tuners to raise the exhaust port. The ESR system is primarily
used for dirt track or kart applications where low-end power is of no
consequence.

Whenever the cylinder is installed on the bottom end after top end
rebuilding, the valves need to be put in the closed position. Otherwise the
HPP cam spindle that connects the actuator in the cases to the cylinder will
get damaged when you tighten down the cylinder. That will also make the
valves inoperable. Always check the HPP valve operation after you assemble
the top end by using the inspection cap on the left side of the cylinder.

The CR125 HPP system was redesigned in 1990. Honda chose to use a system
similar to the 1986-91 CR250, featuring horizontally sliding valves. This
system was plagued with problems over the years. The valves are prone to
carbon seizure because the critical square edges face the exhaust stream. If
the clips that fit on the ends of the valves vibrate off or if the valve
wears too much then the valve can tilt on an angle and strike the piston.
Another common related problem happen when tuners widen the exhaust port
during porting and neglect to grind the valves at the outer corners for
piston clearance. There again the piston strikes the valves because they
protrude into the bore. In 1998 Honda made a modification to the valves,
they added an L-shaped rib that prevented the valves from angling in and
contacting the piston. The other problem of clearance between the top of the
valve and the guide was eliminated so the new style valves provide more
low-end power. These valve and guide sets from the 1998-99 models fit the
CR125 models back to 1990.

In 2000 Honda redesigned the CR125 engine and adapted the exhaust valve
system used on the RS250 roadracer. Honda also used this system on several
dual sport and street bikes sold in Asia and Europe. The new system is so
simple and effective. It is a wedge-shaped valve that pivots at one end,
similar to the CR250. The valve is much thicker and can vary the exhaust
port's effective stroke, time-area, and duration over a wider rpm range.
It's a self-scraping set up so maintenance should be greatly reduced over
previous models.

Kawasaki KIPS

Kawasaki's KIPS exhaust valve system has gone through a steady refinement of
design. Kawasaki uses a different system to suit the needs of the different
model bikes. The earliest KIPS design used two drum shaped valves to control
the flow of the sub exhaust ports. Opening the ports gave the exhaust port
more time-area. The main exhaust port was relatively small with modest
timing and duration. A rack and pinion set up controlled the drum valves,
opening them at about 6,000 rpm. Kawasaki uses the rack and pinion design in
all their KIPS systems except the 1998 and later KX80-125cc models. The 1992
KX125 and KDX used the next generation KIPS which featured a center wedge
valve with two side drum valves engaged to a rack-and-gear actuating system.
This system was very complicated with all its moving parts. The top and
bottom racks had to be synchronized through the left drum valve, which has
two drive gears molded in it. The drum valves are made of aluminum. When the
drum valve becomes carbon seized, the steel teeth on the rack shear off the
aluminum teeth on the drum valve, rendering the drum valve inoperable. Check
the condition on the gear teeth every time you do a top-end service, because
if one gear fails the whole system runs out of sync. . On the late model
80-125cc KXs, the KIPS is relatively simple relying on a wedge valve and
flapper. This system is self-scraping so it requires little maintenance. In
the first year of operation (1998) the KIPS system was plagued with failures
like the pin breaking on the flapper, the valve receding into the cylinder
and contacting the piston, and over-extension of the valve causing cock and
jam. Pro-Circuit made an aftermarket valve cover with a full stop that
prevented over-extension and in 1999 Kawasaki changed the wedge valve and
flapper design for more rigidity and that solved all the reliability
problems.

The drum valves on the 1988- 92 KX250 and 1990-2000 KX500 are also aluminum
but have a hard-anodized coating that resists wear. However, the drum valves
eventually wear at the drive channels for the center wedge valve, and the
sloppy fit between the wedge and drum valves prevents the center valve from
fully opening. That is why these bikes get noticeably slower as they get
older. There is no preventative cure or aftermarket part. You just need to
replace the drum valves when the drive channels wear out. The 1993 KX250 was
the first year for the KIPS system used through present day models. The
system uses a single wedge and flapper valve for the main exhaust port and
two drum shaped valves for the sub exhaust ports. The valves are all linked
together with two racks and pinions on the right drum valve and a steel gear
on the upper rack linking the wedge valve. A left-hand-thread nut retains
the gear to the rod that actuates the wedge valve. Check the nut
periodically, if the nut loosens, the wedge valves become inoperable. The
KX250 KIPS also features two large cavities to allow for dissipation of the
compression wave that travels back up the exhaust pipe at low to mid rpm.
It's important that the two valve covers on the cylinder be sealed with
gaskets and it is normal for large amounts of black sludge to accumulate
under those valve covers. It takes years for the sludge to accumulate to the
point of adversely effecting performance. The only way to clean out the
sludge is to have the cylinder hot-tank cleaned at an automotive rebuilding
store. The 1993–2000 KX250 wedge valve tends to form burrs at the outer
edges that face the piston. These burrs prevent the wedge valve from opening
fully, and the thin flap that comprises the exhaust-port roof hangs out into
the exhaust-gas stream, producing a shock wave that closes off the exhaust
port. File the burrs smooth and check the wedge valve through the full range
of movement. The valve pocket in the cylinder gets worn too. Aftermarket
cylinder rebuilders like US Chrome apply a hard coating to that area to
reduce wear or build-up material that has worn down from the moving wedge
valve. Another characteristic problem of the KX250 KIPS is broken governor
levers. The lever that transmits the movement from the centrifugal governor
to the right-side case lever tends to break in half. This piece is located
under the right side cover. If your KX250 suddenly loses top end power, its
probably due to the actuating lever breakage or the carbon-seizure of the
KIPS valves.

1988–92 KX250 and 1990-2000 KX500 KIPS Timing Procedure

The explanation of this procedure, written in the Kawasaki service manual,
is confusing. It requires you to time the upper and lower racks at the same
instant. My method of timing the exhaust valves is composed of simple steps
that enable you to check your work as you go. The 1988–92 KX250 and KX500
use the drive-channel system to actuate the center valve. Here is the best
way to time the KIPS on these models.

1. Set the cylinder upside down on a bench.

2. Install the center valve but don't bolt it in!

3. Install the side drum valves and align the drive channels on the drum
valves with the center valve, but don't bolt it in!

4. Install the side drums valves and align the drive channels on the drum
valves with the engagement pins on the center valve.

5. Lift up the drum valves so the bottoms of the gears are flush with the
cylinder base. Take care not to disengage the center valve.

6. Slide in the rack from either side of the cylinder. Position the rack by
installing the seal pack and pulling the rack out until it bottoms against
the seal pack. This is the full-open position.

7. Drop the drum valves onto the rack so the valves are in the full-open
position. Don't pay attention to alignment dots or marks on the valve or
rack just remember that the valves should be open when the rack is pulled
out and closed when the rack is pushed in.

1992–97 KX125 and 1993-2000 KX250 KIPS Timing Procedure

The system used on the KX125 and KX250 uses both wedge and drum valves with
racks. This is the best exhaust valve system for performance but the most
difficult to maintain. Here are some tips for re-timing this KIPS system.

1. Install the wedge valves in the cylinder and the actuating rod and lever.
Squirt some pre-mix oil on the parts.

2. Pull the wedge valve into the full-open position, place the gear on the
end of the rod, and rotate the gear counterclockwise until the rack butts
against the stop plate. Thread the nut on the rod and tighten it
counterclockwise because it is a left-hand-thread nut.

3. Place the drum valves into their respective cavities until the top of the
gears are level with the cylinder base. Now push the lower rack into place
and bolt the seal pack on the rack into the cylinder.

4. Pull the rack out until it stops and push it in one millimeter; now it is
in the correct position to install the drum valve. Before you push down the
drum valves, make sure the wedge valve and drum valves are in the full-open
position.

5. Push down the drum valve with the two gears first because it must engage
the upper rack and lower rack simultaneously. Take care and be patient. You
may have to wiggle the wedge valve yoke to get everything to fall into
place. Never hammer the drum valves! Then push down the right drum valve and
install the idler gear. Now install the bushings and check the system. The
valves will bind and stick if you try to move the valves without the
bushings installed, or if the cylinder is facing upside-down. Test the KIPS
in this way, pull the rack outward until it stops, look through the exhaust
port from the pipe side. The valves should be in the full open position. On
cylinders where the base has been turned down more than .010 inches, the
drum valve bushings will also need to be turned down to prevent the valves
from binding when the cylinder is tightened.

Suzuki ATEV

Suzuki first used exhaust valves in 1985, using a drum valve that uncovered
a cavity in the head or cylinder to add volume and length to the exhaust
pipe, strictly at low rpm. In 1987 they employed a system that featured two
large valves that had multiple functions. This system was used on the
1989-2000 RM80, 1987-2000 RM125, 1987-95 RM250. The wedge shaped valves was
positioned at about a 45-degree angle over the exhaust port. The ATEV system
is designed to regulate the effective stroke, exhaust-gas velocity through
the exhaust port, and on 1995 and later models it controls the exhaust gas
temperature. The ATEV system is self-cleaning in that the valves are scraped
of carbon every time they move. Some of the early-model RMs suffered from
broken exhaust valves when the stem would detach from the cylindrical wedge.
That problem was cured in 1991 when the radius between the stem and valve
was increased. The two common problems that occur with the ATEV are caused
by the two following errors in assembling the system:

1. Too much preload on the spring. On the left side of the cylinder is a
dial that controls the spring preload for the exhaust valve system. The
preload doesn't have that great of an affect on the engine's powerband, but
too much preload will prevent the valves from opening, which causes a lack
of top-end power.

2. Crisscrossed spring. A centering spring on the right side of the
cylinder, located on the rod, actuates the valves. This spring is commonly
installed wrong. The spring tabs should be parallel when coupled to the
lever and rod. If the spring tabs are crisscrossed, the valve travel will be
limited and won't open fully.

In 1996 Suzuki redesigned the RM250 engine, going back to a design
reminiscent of the 1987 model RM250. For this model Suzuki modified Honda's
HPP design used on the late model CR250. However a problem plagued this
system. Instead of pivoting the center valve, Suzuki chose to slide it in a
passageway of the cylinder. The added mechanical friction made the system
prone to binding in one position, half-open. This causes the engine to run
flat. Another problem was the shape of the valve. The leading edge that
faced the piston was too square and sharp. Even when the valve was in the
full open position it caused a shock wave that impeded the outgoing exhaust
flow. Grinding the edge smooth reduced the low-end power but helped improve
top end. In 1997 Suzuki redesigned the center valve, choosing steel as a
material and splitting the valve into two sections, a major and minor valve.
They also added a two-stage spring system. With some simple grinding to
match the valve to the exhaust port when fully open, this set up was a
winner! Suzuki chose to redesign the system in 1998-2000 to the 1997 design.
The thought was that the steel valve damaged the valve pocket in the
cylinder. Although simply extending the nickel silicon carbide bore material
into the valve pocket would've solved this problem.

Yamaha POWERVALVE

Yamaha was the first motorcycle manufacturer to adapt exhaust valves to
two-stroke motorcycle engines. Yamaha's simple design of a cylindrical valve
that rotates 1/4 turn to vary the height of the exhaust port requires little
maintenance. This system was used on the YZ250 from 1982-98, and on the
YZ125 from 1983-93. Occasionally, you have to replace the seals and O-rings
to prevent exhaust oil from drooling out of the side if the cylinder. In
1989, Yamaha added a stop plate to limit the travel of the power valve,
primarily so mechanics couldn't install the valve in the wrong position. The
stop plate is located on the left side of the cylinder. The valve has a
small tab that bumps up against the stop plate to limit the fully open and
closed position of the valve. This design enabled Yamaha to position the
valve closer to the piston to make it more effective at varying the
exhaust-port timing. Unfortunately, the soft-aluminum tab on the valve gets
worn, allowing the valve to rotate farther in the fully closed position.
Eventually, (after about three years' use) the tab wears enough so the valve
strikes the piston, causing damage to the piston. Yamaha's exhaust valve is
cheap to replace. I recommend replacing the valve when the tab wears more
than 0.030 inch (0.7mm).

In 1994 Yamaha changed the engine design of the YZ125 and included the next
generation of exhaust valves. This system used two oval-shaped wedge valves,
positioned at a 45-degree angle over the exhaust port. This system was
similar to the one employed by Suzuki. Yamaha experimented with resonator
cavity volume, and vents for pressure bleed off and temperature control.
Overall this is a very reliable system. Occasionally the pins that fit
through the ends of the valve to interface with the actuator lever vibrate
out causing the valve to strike the piston. Those pins are a press fit but
you can add some Loctite Instant Adhesive to the pins for added protection.
One problem that Yamaha is concerned with is high rpm valve flutter. They've
added springs to the valves to control the flutter but future innovations
could include a positive seal between the valve and the cylinders' valve
pocket.

In 1999 Yamaha redesigned the YZ250 engine and exhaust valve system. This
model features a powervalve that marks a significant design change, from the
company that pioneered the use of exhaust valves on two-stroke engines.
Looking more like a Rube Goldberg device, the new powervalve has separate
valves for the main (center) and sub-exhaust ports (sides). The whole
assembly is controlled by one actuating rod, but the side valves open after
the main exhaust valve. The side valves are controlled by two wedge-shaped
ramps that resemble a shift drum from a transmission. The ramp design offers
versatility in tuning. By changing the shape of the ramp, the duration and
timing of the sub exhaust ports can be changed to match a rider's ability or
the demands of the terrain. So far there are no aftermarket companies making
these ramps but you can bet that the factory teams are experimenting with
them!

The new exhaust valve system also features an atmospheric vent that not only
relieves the pipe pressure wave at low rpm but also introduces cool air into
the exhaust system to make the pipe work over a larger rpm range.

KTM 250, 300, 360, 380 1990-97

KTM uses two distinctly different designs of exhaust valves systems. The
earlier model uses one large center valve with the actuating rod cast
together. Two drum valves control the sub exhaust ports and steel gears are
used to interface the main and minor valves. The effective stroke can be
adjusted by altering a stop plate on the left side of the cylinder. The
governor control in the right side case has an inspection cap that allows
tuners to add thin washers and increase the spring preload in order to
affect a change in engagement rpm. The system is prone to carbon seizure of
the steel valves (sub exhaust ports). Also there are rubber o-rings that
prevent oil from leaking out the sides of the actuating rod. Those
eventually wear out. In order to service this exhaust valve system you need
to remove the cylinder. The main valve and all it's hardware can remain
bolted together. There is an access cover on the front of the cylinder and
four bolts fasten the cover to the cylinder. There is no Gasket for the
cover, it seals with a non-drying liquid gasket like RTV silicone. The main
valve just pulls straight out. The drum valves are held in place by two
plates with two-tapered pan head Phillips screws. There is a specific
procedure for removing these screws. Start by heating the screw heads for
two minutes with a propane torch. That will break down the locking agent on
the threads. Then use hand impacts with a number two Phillips tip. If you
strip the heads of the screws (most people do including me!) then just use a
tapered-point punch to spin the screws out. That will destroy the screw but
you should replace the Phillips with a tapered-panhead Allen bolt. Take care
when handling the stop lever on the left side of the main valve. Before you
loosen the two Allen bolts, scribe a line to reference the position of the
plate relative to the gear plate. It is possible to adjust the stop plate so
far that the valve rotates past the full closed position and contacts the
piston. That would destroy the piston. The 1998 and newer KTMs have an
exhaust valve system that makes use of the resonator concept. That system is
so complex that you would need the factory service manual in order to
service it.

BASIC TWO-STROKE TUNING

By Eric Gorr

Changing the power band of your dirt bike engine is simple when you know the
basics. A myriad of different aftermarket accessories is available for you
to custom tune your bike to better suit your needs. The most common mistake
is to choose the wrong combination of engine components, making the engine
run worse than stock. Use this as a guide to inform yourself on how changes
in engine components can alter the powerband of bike's engine. Use the guide
at the end of the chapter to map out your strategy for changing engine
components to create the perfect power band.

TWO-STROKE PRINCIPLES

Although a two-stroke engine has less moving parts than a four-stroke
engine, a two-stroke is a complex engine because it relies on gas dynamics.
There are different phases taking place in the crankcase and in the cylinder
bore at the same time. That is how a two-stroke engine completes a power
cycle in only 360 degrees of crankshaft rotation compared to a four-stroke
engine which requires 720 degrees of crankshaft rotation to complete one
power cycle. These four drawings give an explanation of how a two-stroke
engine works.

1) Starting with the piston at top dead center (TDC 0 degrees) ignition has
occurred and the gasses in the combustion chamber are expanding and pushing
down the piston. This pressurizes the crankcase causing the reed valve to
close. At about 90 degrees after TDC the exhaust port opens ending the power
stroke. A pressure wave of hot expanding gasses flows down the exhaust pipe.
The blow-down phase has started and will end when the transfer ports open.
The pressure in the cylinder must blow-down to below the pressure in the
crankcase in order for the unburned mixture gasses to flow out the transfer
ports during the scavenging phase.

2) Now the transfer ports are uncovered at about 120 degrees after TDC. The
scavenging phase has begun. Meaning that the unburned mixture gasses are
flowing out of the transfers and merging together to form a loop. The gasses
travel up the back side of the cylinder and loops around in the cylinder
head to scavenge out the burnt mixture gasses from the previous power
stroke. It is critical that the burnt gasses are scavenged from the
combustion chamber, in order to make room for as much unburned gasses as
possible. That is the key to making more power in a two-stroke engine. The
more unburned gasses you can squeeze into the combustion chamber, the more
the engine will produce. Now the loop of unburned mixture gasses have
traveled into the exhaust pipe's header section. The gasses aren't lost
because a compression pressure wave has reflected from the end of the
exhaust pipe, to pack the unburned gasses back into the cylinder before the
piston closes off the port. This is the unique super-charging effect of
two-stroke engines. The main advantage of two-stroke engines is that they
can combust more volume of fuel/air mixture than the swept volume of the
engine. Example: A 125cc four-stroke engine combusts about 110cc of F/A
gasses but a 125cc two-stroke engine combusts about 180cc of F/A gasses.

3) Now the crankshaft has rotated past bottom dead center (BDC 180 degrees)
and the piston is on the upstroke. The compression wave reflected from the
exhaust pipe is packing the unburned gasses back in through the exhaust port
as the piston closes off the port the start the compression phase. In the
crankcase the pressure is below atmospheric producing a vacuum and a fresh
charge of unburned mixture gasses is flowing through the reed valve into the
crankcase.

4) The unburned mixture gasses are compresses and just before the piston
reaches TDC, the ignition system discharges a spark causing the gasses to
ignite and start the process all over again.

CYLINDER PORTING

The cylinder ports are designed to produce a certain power characteristic
over a fairly narrow rpm band. Porting or tuning is a metal machining
process performed to the cylinder ports (exhaust & transfers) that alters
the timing, area size, and angles of the ports in order to adjust the power
band to better suit the rider's demands. For example, a veteran trail rider
riding an RM250 in the Rocky mountain region of the USA will need to adjust
the power band for more low end power because of the steep hill climbs and
the lower air density of higher altitudes. The only way to determine what
changes will be needed to the engine is by measuring and calculating the
stock engine's specifications. The most critical measurement is termed
port-time-area. This term is a calculation of a port's size area and timing
in relation to the displacement of the engine and the rpm. Experienced
tuners know what the port-time-area values of the exhaust and transfer ports
should be for an engine used for a particular purpose. In general, if a
tuner wants to adjust the engine's power band for more low to mid range he
would do the following things. Turn down the cylinder base on a lathe to
increase the effective stroke (distance from TDC to exhaust port opening).
This also retards the exhaust port timing and shortens the duration and
increases the compression ratio. Next the transfer ports should be narrowed
and re-angled with epoxy to reduce the port-time-area for an rpm peak of
7,000 rpm. The rear transfer ports need to be re-angled so they oppose each
other rather than pointing forward to the exhaust port. This changes the
loop scavenging flow pattern of the transfer ports to improve scavenging
efficiency at low to mid rpm (2,000 to 5,000 rpm). An expert rider racing mx
in England would want to adjust the power band of an RM250 for more mid to
top end power. The cylinder would need to be tuned radically different than
for trail riding.

Here is an example. The exhaust port would have to be raised and widened to
change the port-time-area peak for a higher rpm (9,000 rpm). For either of
these cylinder modifications to be effective, other engine components would
also need to be changed to get the desired tuning effect.

CYLINDER HEAD

Cylinder heads can be reshaped to change the power band. Generally speaking,
a cylinder head with a small diameter and deep combustion chamber, and a
wide squish band (60% of the bore area). Combined with a compression ratio
of 9 to 1 is ideally suited for low to mid range power. A cylinder head with
a wide shallow chamber and a narrow squish band (35-45% of bore area) and a
compression ratio of 8 to 1, is ideally suited for high rpm power.

There are many reasons why a particular head design works for certain types
of racing. For example; a head with a wide squish band and a high
compression ratio will generate high turbulence in the combustion chamber.
This turbulence is termed Maximum Squish Velocity, MSV is rated in meters
per second (m/s). A cylinder head designed for supercross should have an MSV
rating of 28m/s. Computer design software is used to calculate the MSV for
head designs. In the model tuning tips chapters of this book, all the head
specs quoted have MSV ratings designed for the intended power band changes.

CRANKSHAFT

There are two popular mods hop-up companies are doing to crankshafts;
stroking and turbo-vaning. Stroking means to increase the distance from the
crank center to the big end pin center. There are two techniques for
stroking crankshafts; weld old hole and re-drill a new big end pin hole, or
by installing an off-set big end pin. The method of weld and re-drilling is
labor intensive. The off-set pin system is cheap, non-permanent, and can be
changed quickly. In general, increasing the stroke of a crankshaft boosts
the mid range power but decreases the engine's rpm peak.

The term "Turbo-Crank" refers to a modification to the crankshaft of a
two-stroke engine, whereby scoops are fastened to the crank in order to
improve the volumetric efficiency of the engine. Every decade some hop-up
shop revives this old idea and gives it a trendy name with product promises
that it can't live up to. These crank modifications cause oil to be directed
away from the connecting rod and often times the vanes will detach from the
crank at high rpm, causing catastrophic engine damage. My advice, don't
waste the $750!

CARBURETOR

In general a small diameter carburetor will have high velocity and a good
flow characteristic for a low to mid rpm power band. A large diameter
carburetor works better for high rpm power bands. For 125 cc engines a 34mm
carburetor works well for supercross and enduro and a 36 or 338 mm
carburetor works best for fast mx tracks. For 250 cc engines a 36 mm
carburetor works best for low to mid power bands and a 39.5 mm carburetor
works best for top end power bands. Recently there has been a trend in the
use of air-foils and rifle-boring for carbs. These innovations are designed
to improve air flow at low throttle openings. Some companies sell carb
inserts, to change the diameter of a carb. Typically a set of inserts is
sold with a service of over boring the carb. For example; a carb for a 250cc
bike (38mm) will be bored to 39.5mm and two inserts will be supplied. The
carb can then be restricted to a diameter of 36 or 38mm.

REED VALVE

Think of a reed valve like a carburetor, bigger valves with large flow-areas
work best for high rpm power bands. In general, reed valves with six or more
petals are used for high rpm engines. Reed valves with four petals are used
for dirt bikes that need strong low end and mid range power. There are three
other factors to consider when choosing a reed valve. The angle of the reed
valve, the type of reed material, and the petal thickness. The two common
reed valve angles are 30 and 45 degrees. A 30-degree valve is designed for
low to mid rpm and a 45 degree valve is designed for high rpm. There are two
types of reed petal materials commonly used, carbon fiber and fiberglass.
Carbon fiber reeds are lightweight but relatively stiff (spring tension) and
designed to resist fluttering at high rpm. Fiberglass reeds have relatively
low spring tension so they instantly respond to pressure that changes in the
crankcase, however the low spring tension makes them flutter at high rpm
thereby limiting the amount of power. Fiberglass reed petals are good for
low to mid power bands and carbon fiber reeds are better for high rpm
engines.

Boyesen Dual Stage reeds have a large thick base reed with a smaller thinner
reed mounted on top. This setup widens the rpm range where the reed valve
flows best. The thin reeds respond to low rpm and low frequency pressure
pulses. The thick reeds respond to higher-pressure pulses and resist
fluttering at high rpm. A Boyesen RAD valve is different than a traditional
reed valve. Bikes with single rear shocks have off-set carbs. The RAD valve
is designed to redistribute the gas flow to the crankcases evenly. A RAD
valve will give an overall improvement to the power band. Polini of Italy
makes a reed valve called the Supervalve. It features several mini sets of
reeds positioned vertically instead of horizontally like conventional reed
valves. These valves are excellent for enduro riding because of improved
throttle response. In tests on an inertia chassis dyno show the Supervalve
to be superior when power shifting. However these valves don't generate
greater peak power than conventional reed valves. Supervalves are imported
to America and sold by Moto Italia in Maine.

EXHAUST PIPE

The exhaust pipe of a two-stroke engine attempts to harness the energy of
the pressure waves from combustion. The diameter and length of the five main
sections of a pipe, are critical to producing the desired power band. The
five sections of the pipe are the head pipe, diffuser cone, dwell, baffle
cone, and the stinger. In general, after market exhaust pipes shift the
power band up the rpm scale. Most pipes are designed for original cylinders
not tuned cylinders. Companies like MOTOWERKS custom computer design and
fabricate pipes based on the cylinder specifications and the type of power
band targeted.

SILENCER

Silencers come in all sorts of shapes and sizes. A long silencer with a
small diameter enhance the low to mid power because it increases the
bleed-down pressure in the pipe. A silencer with a short length and a large
core diameter provides the best bleed-down pressure for a high rpm engine.
Too much pressure in the pipe at high rpm will radically increase the
temperature of the piston crown and could cause the piston to seize in the
cylinder.

FLYWHEEL WEIGHTS

The flywheel is weighted to improve the engine's tractability at low to mid
rpms. There are two different types of flywheel weights, weld-on and
thread-on. A-Loop performs the weld-on flywheel weight service. Steahly
makes thread-on flywheel weights. This product threads onto the fine
left-hand threads that are on the center hub of most Japanese magneto
rotors. normally the threads are used for the flywheel remover tool.
Thread-on flywheel weights can only be used if the threads on the flywheel
are in perfect condition. The advantage to weld-on weights is they can't
possibly come off.

External rotor flywheels have a larger diameter than internal rotor
flywheels so they have greater flywheel inertia. Internal rotor flywheels
give quicker throttle response.

AFFECTS OF THE IGNITION TIMING

Here is how changes in the static ignition timing affects the power band of
a Japanese dirt bike. Advancing the timing will make the power band hit
harder in the mid range but fall flat on top end. Advancing the timing gives
the flame front in the combustion chamber, adequate time to travel across
the chamber to form a great pressure rise. The rapid pressure rise
contributes to a power band's "Hit". In some cases the pressure rise can be
so great that it causes an audible pinging noise from the engine. As the
engine rpm increases, the pressure in the cylinder becomes so great that
pumping losses occur to the piston. That is why engines with too much spark
advance or too high of a compression ratio, run flat at high rpm.

Retarding the timing will make the power band smoother in the mid-range and
give more top end over rev. When the spark fires closer to TDC, the pressure
rise in the cylinder isn't as great. The emphasis is on gaining more degrees
of retard at high rpm. This causes a shift of the heat from the cylinder to
the pipe. This can prevent the piston from melting at high rpm, but the
biggest benefit is how the heat affects the tuning in the pipe. When the
temperature rises, the velocity of the waves in the pipe increases. At high
rpm this can cause a closer synchronization between the returning
compression wave and the piston speed. This effectively extends the rpm peak
of the pipe.

HOW TO ADJUST THE TIMING

Rotating the stator plate relative to the crankcases changes the timing.
Most manufacturers stamp the stator plate with three marks, near the plate's
mounting holes. The center mark is the standard timing. If you loosen the
plate mounting bolts and rotate the stator plate clockwise to the flywheel's
rotation, that will advance the ignition timing. If you rotate the stator
plate counterclockwise to the flywheel's rotation, that will retard the
ignition timing. Never rotate the stator plate more than .028in/.7mm past
the original standard timing mark. Kawasaki and Yamaha stator plates are
marked. Honda stators have a sheet metal plate riveted to one of the mount
holes. This plate insures that the stator can only be installed in one
position. If you want to adjust the ignition timing on a Honda CR, you'll
have to file the sheet metal plate, with a 1/4in rat-tail file.

AFTERMARKET IGNITIONS

The latest innovation in ignition systems is an internal rotor with bolt-on
discs that function as flywheel weights. PVL of Germany makes these
ignitions for modern Japanese dirt bikes. Another advantage to the PVL
ignition is that they make a variety of disc weights so you can tune the
flywheel inertia to suit racetrack conditions.

MSD is an aftermarket ignition component manufacturer. They are making
ignition systems for CR and RM 125 and 250. MSD's ignition system features
the ability to control the number of degrees of advance and retard. These
aftermarket ignition systems sell for less than the OEM equivalent.

TIPS FOR BIG BORING CYLINDERS

In the mid nineties, European electro-plating companies started service
centers in America. This made it possible to over bore cylinders and
electro-plate them to precise tolerances. This process is used by tuners to
push an engine's displacement to the limit of the racing class rules, or
make the engine legal for a different class.

When you change the displacement of the cylinder, there are so many factors
to consider. Factors like; port-time-area, compression ratio, exhaust
valves, carb jetting, silencer, and ignition timing. Here is an explanation
of what you need to do when planning to over bore a cylinder.

Port-Time-Area - This is the size and opening timing of the exhaust and
intake ports, versus the size of the cylinder and the rpm. When increasing
the displacement of the cylinder, the cylinder has to be bored to a larger
diameter. The ports enter the cylinder at angles of approximately 15
degrees. When the cylinder is bore is made larger, the transfer ports drop
in height and retard the timing and duration of those ports. The exhaust
port gets narrower. If you just over bored and plated a cylinder, it would
have much more low end power than stock. Normally tuners have to adjust the
ports to suit the demands of the larger engine displacement. Those exact
dimension changes can be determined with TSR's Time-Area computer program.

Cylinder Head - The head's dimensions must be changed to suit the larger
piston. The bore must be enlarged to the finished bore size. Then the squish
band deck height must be set to the proper installed squish clearance. The
larger bore size will increase the squish turbulence so the head's squish
band may have to be narrowed. The volume of the head must be increased to
suit the change in cylinder displacement. Otherwise the engine will run flat
at high rpm or ping in the mid range from detonation.

Exhaust Valves - When the bore size is increased, the exhaust valve to
piston clearance must be checked and adjusted. This pertains to the types of
exhaust valves that operate within close proximity of the piston. If the
exhaust valves aren't modified, the piston could strike the valves and cause
serious engine damage.

Carb - The piston diameter and carb bore diameter are closely related. The
larger the ratio between the piston size and the carb size, the higher the
intake velocity. That makes the jetting richer. Figure on leaning the
jetting after an engine is over bored.

Ignition Timing - The timing can be retarded to improve the over rev.
Normally over bored engines tend to run flat on top end.

Pipe and Silencer - Because only the bore size is changed, you won't need a
longer pipe only one with a larger center section. FMF's line of Fatty pipes
work great on engines with larger displacement. Some riders use silencers
that are shorter with larger outlets to adjust the back-pressure in the pipe
for the larger engine displacement.

Fuel For Thought

By Rich Rohrich

Part 1 - The Basics

It's getting so you can't read the newsgroups or walk through the pits at a
racetrack without bumping into a group of racers discussing fuel, and the
problems associated with it. It's obvious to everyone that the fuel we use
is a vital link in the performance chain, yet it is surrounded by more than
the usual share of myth and misinformation. Given the sweeping changes on
pump fuels mandated by the government, and the vast number of race gas
offerings the subject is a hot topic. Through this series of articles I hope
to shed a little light and hopefully squish a myth or two along the way.

What does the Octane Number at the pump mean?

MTBE

The octane rating of a fuel is what most people are familiar with, but there
seems to be a lot of confusion surrounding it. In simple terms the octane
number you see at the pump is the average of two octane numbers; the
Research Octane Number (RON) and the Motor Octane Number (MON) or (RON +
MON) / 2. This final octane number is sometimes referred to as the Anti
Knock Index or AKI. This pump octane number is a measure of the anti- knock
characteristics of a given fuel.

MON and RON are determined by standardized ASTM laboratory tests. The
details of the tests are not as important as what they mean in terms of
performance. Low to medium-speed knock characteristics are determined by the
Research (RON) method, while high-speed and partial throttle heavy load
knock characteristics are determined by the Motor (MON) method. MON testing
is conducted under more stringent conditions with the timing on the test
engine advanced and run with a higher inlet air temperature, so the MON
number tends to be lower but also more valid for high-performance
applications. There are a number of more valid tests that have been
developed to determine the anti-knock characteristics of fuels used in high
performance engines, but the aren't in general use at this point so we are
stuck with the old reliable pump octane number.

So what's that knocking sound coming from my engine?

The Knocking sound you hear when your engine is in trouble are the result of
abnormal combustion. The most common combustion problems are detonation and
pre-ignition. In simple terms detonation is the uncontrolled burning of the
fuel in the combustion chamber, while pre-ignition can be defined as the
starting of the burning process by any source other than the spark plug
usually before the plug has fired.

To truly understand what detonation is, its important to understand that if
you raise the temperature of any combustible mixture high enough, it will
ignite on its own. This is sometimes called the "spontaneous combustion
point" or the "auto ignition temperature". Detonation is a rapid
uncontrolled rise in cylinder pressure caused by all or part of the fuel
mixture reaching this auto-ignition temperature.

Following the ignition process through a cycle should help complete the
explanation. As the piston rises and compresses the trapped mixture the
pressure and temperature begins to rise. The spark plug fires somewhere
before the piston reaches Top Dead Center (TDC) and starts burning the
compressed mixture in the cylinder and as a consequence raises the
combustion chamber temperature. While this burning is taking place, the
piston is still rising and still compressing the air/fuel mixture which
raises the cylinder pressure, and combustion chamber temperature even
higher.

At this point, the pressure rise in the cylinder is very rapid, but it
generally proceeds at a fairly even controlled rate. The remaining unburned
mixture and the end gases at the edges of the combustion chamber are being
raised to extremely high temperatures as the advancing flame front
compresses and heats up the mixture directly in front of it. This activity
before the flame front reaches the end gases at the edge of the chamber are
sometimes called pre-flame reactions. The longer it takes for the complete
burning to take place the greater the chances that these pre-flame reactions
will force the end gases to reach the auto ignition point and cause a rapid
uncontrolled pressure rise, along with a huge increase in cylinder
temperature. If brought to the auto ignition point the end gases of the
combustion chamber can cause a pressure and frequency rise that is high
enough to be audible. That's the KNOCK or PING that you hear. Ideally, the
burning of the mixture will be completed before any of these end gases have
an opportunity to reach the point of auto ignition. If the ignition timing
is set correctly this should happen around 15-20 degrees After Top Dead
Center (ATDC).

It's hard to visualize the immense pressures we are talking about in the
combustion chamber. In a normal combustion cycle the pressures can easily
reach 100 times the trapped compression ratio, that's 800-1300 psi banging
away at the piston crown and cylinder head and bearings. Once an engine
starts to detonate the pressures can reach 3 to 4 times that high. The
pressure rise during detonation can be almost instantaneous, so it's easy to
see why the edges of the piston can be broken away during these cycles. It's
like having a small bomb go off in the engine.

As you may have guessed from the earlier discussion of octane numbers, high
octane fuels have a considerably higher auto ignition temperature to keep
these pre-flame reactions from causing sudden uncontrolled pressure rises.
If the charge burns fast enough or the fuel is resistant enough to auto
ignition (high octane) then all is well and the pressure rise isn't too
extreme. Hopefully it should be fairly clear that if you can shorten the
burn time (10% to 90% burned) enough then the octane requirement of the
engine will be reduced. As a general rule, the first half of combustion
0-50% burned, speeds up in direct proportion to rpm, while the 50-100%
burned time speeds up exponentially with rpm. So all other things equal, the
faster you spin an engine, the faster the charge will burn and the more
knock resistant the engine will be. Small bore, high rpm motors are by
design, very knock resistant.

We defined pre-ignition previously as the starting of the burning process by
a source other than the plug. This has the same effect as advancing the
timing. This causes the engine to be subjected to huge amounts of heat,
because the piston and cylinder walls are subjected to the burning process
for a longer period of time, this in turn raises the combustion chamber
pressure. It's possible for pre-ignition to melt the top of pistons because
of the extreme temperatures that this advanced timing causes. Keep in mind
that any time you raise the temperature in the cylinder you get a
corresponding rise in pressure , or conversely raising the pressure also
raises the temperature. So it's easy to see how pre-ignition and detonation
are very closely linked.

So it pretty much boils down to this. If you can control the pressure and
temperature in the combustion chamber , things will go along with out too
many problems. But once you cross that temperature/pressure threshold a
number of interrelated actions can take place that causes all hell to break
lose. High octane fuel is one way to keep the carnage in check.

How much octane do we need?

Cylinder pressure is one of the key factors in determining the octane
requirement of an engine. Intake valve closing time on four-stroke engines,
and exhaust timing on two-strokes will have a major influence on the dynamic
cylinder pressure . It's a commonly held misconception that higher Octane
fuel slows down the flame speed which keeps the engine from knocking. Flame
speed is a function of fuel chemistry, not the Octane rating. The component
make up of the fuel will determine the flame speed whether it's a high
octane fuel or not. . Racing fuels designed for high rpm applications tend
to have higher flame speeds than normal to help reduce burn time. There
isn't much time available to complete the combustion cycle at 10,000rpm, so
choosing the right fuel can really make a difference. Choosing a faster
burning fuel will allow you to run less ignition advance, and ultimately
make more power at higher revs.

Every engine can have radically different requirements. Even two similarly
modified engines can have requirements as different as 5 to 8 MON numbers in
some cases. The factors affecting octane requirements should be of great
interest to every racer. By changing these factors around you can raise or
lower the octane requirement of your engine. Some of the more obvious
factors are :

D E S I G N F A C T O R S O P E R A T I N G C O N D I T I O N S

Compression ratio Outside air temperature/
Intake air temperature

Trapped Compression Ratio Altitude

Ignition timing Humidity

Combustion chamber shape Barometric pressure

Charge Motion in the cylinder Premix ratio (two-strokes)

Air/fuel ratio Engine RPM

Cooling efficiency Engine load

Scavenging efficiency

Spark plug location

Spark plug heat range

In looking at this list, it should be apparent to you that a number of these
factors/conditions are pretty much out of our control. We will concentrate
on those that are more easily changed.

Is it better to raise fuel octane or lower the octane requirement of the
engine? The answer to that question is yes. You want to lower the octane
requirement of the engine as much as possible without lowering engine
performance. You also want to use a fuel with an octane rating just high
enough to keep your engine from ever detonating.

Engine timing is one of the factors on the list, but on most modern engines
the timing is fixed or electronically controlled. You have more to lose than
gain if you play with the timing of these ignitions, it's best to leave them
alone.

Another way to fend off detonation is to make sure the engine's cooling
system works as efficiently as possible. That means clean radiators at all
times, and unrestricted airflow across the cooling fins on air cooled
engines. It's important to keep the water temperature between 50 and 70
degrees Centigrade. This range produces the most power while keeping the
octane requirement low.

Most two-cycle engines have a squish band machined at the outer edge of the
cylinder head, this band serves two purposes . The close proximity of the
piston to the cylinder head helps to cool (or quench) the end gases that
were heated by the pre-flame reactions, but more importantly the squish band
helps to add motion to the burning charge, which helps speed up the burning
and limits the amount of time available for pre-flame reactions to heat the
end gases. A fast burning chamber will tend to be very resistant to
detonation . This is part of the reason you've seen manufacturers switching
from domed pistons, to flat top pistons and back again. They are always
looking for that magic combination of scavenging efficiency and charge
motion. Unfortunately, on most stock engines that squish band doesn't serve
much purpose. In theory squish bands work very well, but production line
tolerances leave squish clearances so great that the only thing being
squished is horsepower. Cutting the cylinder head to bring the squish
clearance into spec while still retaining the original cylinder head shape
and volume is not the easiest thing to do, though it will definitely pay
dividends .

Four-stroke engines usually have a quench area designed into the cylinder
head to aid in lowering the temperature of the end gases, while charge
motion is often generated as a combination of intake port velocity, port
angles, piston shape and combustion chamber shape. We'll go into the
specific interactions in a future part of this series. Large bore engines
tend to have a higher octane requirement then smaller bore engines given the
same compression ratio, because of the longer distance the flame front has
to travel and the additional time available for pre-flame reactions to take
place. It is possible to use two spark plugs per cylinder to lower your
engine's octane needs in large bore engines. Adding a second spark plug
shortens the fuel burn time and decreases the distance the flame front needs
to travel.

One of the single most important things you can do to lower the octane
requirement and save your engine some self destruction via the detonation
express is to LEARN TO JET! Running too lean causes excessive heat build up
that can pave the way for Detonation and Pre-ignition. Check out the
Technical Articles section for a good jetting tutorial.

Benzene Toluene

This all brings us back to the octane question : How much do you need. We'll
go into more detail in Part II of the series but here are a few suggestions
to get you started:

Start simple and work your way up. Try a good grade of premium gas that
doesn't contain ANY ALCOHOL. In most states they will have a sign on the
side of the pump warning you about the percentage of alcohol (ethanol or
methanol) in the fuel. Most well modified normally aspirated engines can run
on 95 -100 octane gasoline. Good porting with flow matched transfer ports
can significantly lower octane requirement on two cycle engines. If your
engine detonates try one (or all) of the measures to lower the octane
requirement of the engine. If all these measures fail, try mixing pump gas
50/50 with a good of good quality race gas (Phillips 66, Power Mist, UNION
76, Sunoco, VP, ELF, etc..) with your gasoline. Make sure you use race gas
specifically designed for you your type of application. The fuel
manufacturers can make recommendations based on your engines rpm range, bore
size, and the type of riding you'll be doing. It's best to stay away from
AvGas for your bike. We'll go into the specifics of AvGas in the next
installment. Keep in mind that octane requirement is lower at high altitude
and high humidity. An engine that ran fine at 10,000 feet could very easily
detonate at sea-level, or a sudden drop in humidity on a hot day can cause
knocking that never appeared before.

Appendix A - Fuel Terms glossary

Coming up in part II :

You guys who think that you don't need race gas just because your engine
doesn't detonate on pump gas or some other home brewed combo, are missing
the boat (a boat filled with throttle response, and maybe HP). The idea of
running emissions legislated pump gas designed for engines that turn maybe
4000 rpm in a 10,000 rpm race motor is short-sighted at best. This might
have been OK 10-12 years ago, but nowadays with the economics and
legislation involved in mass market fuel production, it's a bad idea. There
is a hell of a lot more to race gas then just an Octane number. Most good
race gas producers have a variety of fuels available so you can match a fuel
type to your application. Picking the right fuel can make a huge difference
in throttle response and jetting consistency.

We'll go into the details of comparing race gas to find the right fuel for
your application, and we'll also tell you why AVGAS is a poor choice for
your race bike, and we'll give you some rules of thumb for determining your
bikes octane needs. Plus we'll look at the specifics of the new federally
mandated Reformulated fuels, Oxygenated fuels, and a bunch of other trick
stuff.

Send your complaints, corrections or questions to rroh...@interaccess.com .

STAY TUNED!

References and further reading :

Harold H. Schobert - The Chemistry of Hydrocarbon Fuels -
Butterworth-Heinemann Ltd.

Keith Owen, Trevor Coley - Automotive Fuels Reference Book - SAE - R151

H.P. Lenz - Mixture Formation in Spark-Ignition Engines - Springer-Verlag

Jeff Hartman - Fuel Injection - Motorbooks International

Germane, Wood, Hess - Lean Combustion in Spark-Ignited Internal Combustion
Engines - A Review - SAE paper 831694

Z. Warhaft - An Introduction to Thermal Fluid Engineering - Cambridge
University Press

Copyright © 2002 Richard Rohrich
All Rights Reserved

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[Brian McGarry]

Jeff wrote and I snipped 2384 lines:

I know all that.

Brian

[Jeff]

Glad your so smart, others may not be as smart as you! Ill make sure to
start out with, not for super smart people, only dumbies who are learning
the sport.

Jeff

"Brian McGarry" <sca...@execpc.com> wrote in message
news:3d174b38$0$1426$272e...@news.execpc.com...

> Jeff wrote and I snipped 2384 lines:
>
> I know all that.
>
> Brian
>
>

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[David Levy]

I'm a hell of a lot smarter than him.

"Jeff" <fal_...@NOSPAMhotmail.com> wrote in message
news:azIR8.164802$6m5.1...@rwcrnsc51.ops.asp.att.net...

[Brian McGarry]

"Jeff" <fal_...@NOSPAMhotmail.com> wrote in message

> Glad your so smart, others may not be as smart as you! Ill make sure to

> start out with, not for super smart people, only dumbies who are learning
> the sport.

Gosh darn, you don't have to be very smart to know all that stuff.
Want prove? check out who the author is.

Brian

[David Levy]

Check out who is the author.
<grammar police #59300300>

"Brian McGarry" <sca...@execpc.com> wrote in message

news:3d176751$0$3579$272e...@news.execpc.com...
><snip>

[Simone]

Thank you very much, there are a lot of people who surely appreciated it. I
am one of those, though I didn't read it, but I surely will whenever I've
got some time and/or need to do something in the bike.

Simone

Jeff <fal_...@NOSPAMhotmail.com> wrote in message

news:sxyR8.133221$R61....@rwcrnsc52.ops.asp.att.net...

[DirtCrashr]

Yo! Check da 'aight-uh.

DroopBaggyPants

>Check out who is the author.
><grammar police #59300300>

>"Brian McGarry"

>><snip>
>>check out who the author is.
>>
>> Brian

'97 KTM 300 MXC, '99 Beta Techno 280 - D36, BRC, CERA, COHVCO, CORVA. etc.

[Bobby]

Thanks for the information Jeff. There are many riders out there who
benefit from folks like you willing to take the time to research these
things and then respond to a post.

Please continue!

Sincerely,
Bobby

[John Arnett]

Jeff, thanks for the info. I printed it and put it in the manual. It's
easier than trying to look it up in my manual, since it covers, several
models and several carbs, etc. I can appreciate the time it took you to
write it. Thanks again.
John

"Jeff" <fal_...@NOSPAMhotmail.com> wrote in message
news:sxyR8.133221$R61....@rwcrnsc52.ops.asp.att.net...

> Heres all I could find on 2 stroke dirt bikes. i hope this gives you some
> good bathroom material...
>
> Jeff

[snip]

[Mike W.]

On Mon, 24 Jun 2002 13:45:16 -0500, "David Levy" <dl...@pobox.com> wrote:

>Check out who is the author.
><grammar police #59300300>

<Humor police #ZZZZZzzzzzzzzzz>

--
Mike W.
96 XR400
74 CZ250 Enduro
BRC, AMA, NETRA, NOHVCC, NRA

Suburban trail-riding best practices:
http://www.crocker.com/~mwilliams/Suburban.htm

[John Arnett]

Was that a dangling participle?

"David Levy" <dl...@pobox.com> wrote in message
news:af7p8d$bqnve$1...@ID-132159.news.dfncis.de...

[Jay C]

"John Arnett" <jar...@barefootpi.com; mec...@aol.com> wrote in message
news:q3LR8.5609$wR.4...@twister.southeast.rr.com...

> Was that a dangling participle?

I think is that punching-bag thing that hangs down in the back of your
throat.

Jay

[Wudsracer]

On Wed, 26 Jun 2002 11:44:05 -0400, "Jay C" <jwc.N...@sysmatrix.net>
wrote:

That is your uvula.
Wudsracer
'99 Gas Gas EC 274
Team LAGNAF
SMACKOVER Racing
www.smackovermotorsports.com

"We Only Ride on Days That End in Y"