Hesitation from low rpm / full throttle
Dennis
leaking a small ammount gas from the carbs when leaning on the kickstand
after riding. I rebuilt the carbs and set the float levels and air/idle mix
to the factory specs. The bike runs real strong and idles perfectly.
I have noticed since it has gotten cooler ( 60 deg F ) the bike has a
stumble going from low throttle to full throttle when driving. It is most
pronounced when crusing under 2000 rpms and suddenly given full throttle.
When I do this, the engine stumbles for about a seconds before pulling
strongly. When the temperature outside is above 80 deg, the stumble
dissappears.
I have tried opening the air/idle mixture screw a bit in the cooler weather,
but it doesn't seem to make any difference other than at idle.
Would syncing the carbs eliminate the stumble? Any other suggestions?
Who Me?
"Dennis" <cctden@sbc_global.net> wrote
> Would syncing the carbs eliminate the stumble? Any other suggestions?
>
Maybe but probably not.
Running some good carb cleaner through, to get what you missed in the tiny
passages, might help.
Original Gumout Carb and Choke cleaner or Berryman's B12 are good ones. Put
twice the recommended amount in 1/2 tank of gas. If/when it starts to get
better, fill the tank the rest of the way.
And, yes, that mixture screw you have been turning is only for (near) idle.
HOWEVER............
I think the real key here is that you are trying to make it do something
it's probably NOT going to do.
Whipping the throttle WFO from 2K is asking a lot from a 25 year old
engine.......unless you are in first gear maybe. I'll bet it "jumps" just
fine from 4k........or maybe even if you take a full second or so to get to
WFO from 2K, instead of .2 seconds.
It is, after all, not a drag racer; wasn't when it was new. ;-)
Dennis
"Who Me?" <hitch...@dont.panic> wrote in message
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When I rebuilt the carb I soaked it in carb cleaner for a few hours, so I
doubt carb cleaner would do any good.
Maybe I'm just expecting "perfect" throttle response, which is probably
unrealistic for a 25 yr old bike.. I am pretty impressed, as it does pull
away from my friend's 04 Sabre 1100!! Go figure!
.
> I have noticed since it has gotten cooler ( 60 deg F ) the bike has a
> stumble going from low throttle to full throttle when driving. � It is most
> pronounced when crusing under 2000 rpms and suddenly given full throttle.
There are usually three acceleration transition ports right where the
bottom of the throttle butterflies touch the bottom of the carburetor
body.
When the butterflies are closed, you can see *half* of one of the
holes, and that half a hole, combined with the single idle mixture
orifice about an inch further downstream supply *all* the idle mixture
you need when the butterflies are closed.
Now, when you just crack the throttle open a smidgen, the engine
vacuum drops off and there isn't enough vacuum to raise the vacuum
slide and it wouldn't matter if the slide would raise a quarter of an
inch or so when you twisted the throttle, because the upper portion of
the needle isn't tapered enough to draw fuel up through the needle
jet.
When you open the throttle less fuel air mixture is drawn through the
idle mixture port, but more mixture is drawn through the three
transition ports.
Adjusting the idle mixture properly requires that the mechanic know
about the transition ports and how they affect mixture just off idle.
If you have adjusted the idle mixture screws too far open, and then
adjusted the idle speed screw to compensate for an overly rich
mixture, the idle speed will race up to 3000 or 4000 RPM, especially
when the engine gets hot.
Soaking the carbs won't clean out the idle mixture passages, you have
to squirt aerosol carb cleaner through the pilot air jet and watch it
squirt vigorously out the pilot jet, the three transition ports and
the single idle mixture orifice.
And, it's necessary to put your fingers over some of the orifices to
get enough pressure to vigorously flush out those passages.
If I'm wrong, and all those passages are pefectly from your previous
efforts, you may have a tiny tear or pinhole leak in one of the vacuum
diaphragms.
Also, check the carburetor parts fiche at www.bikebandit.com to see if
there was a small rubber o-ring on top of the carburetor to allow the
engine to suck the air out of the vacuum chamber.
Those small o-rings sometimes get lost in the cleaning process and the
mechanic cannot understand why the vacuum slides don't lift...
Dennis
>If you have adjusted the idle mixture screws too far open, and then
>adjusted the idle speed screw to compensate for an overly rich
>mixture, the idle speed will race up to 3000 or 4000 RPM, especially
>when the engine gets hot.
I currently curretly have them set at 2 1/2 turns out. If I open them to 3,
the idle is rougher and the hesitation is reduce. At 3, the bike idles at
~900 RPM a couple minutes after it is started and ~1200 when hot.
>If I'm wrong, and all those passages are pefectly from your previous
>efforts, you may have a tiny tear or pinhole leak in one of the vacuum
>diaphragms.
I was pretty vigerous in the cleaning. Soaked, sprayed, let sit, sprayed
again, compessed air... I looked on the fiche and I didn't see any
diagrapham.
http://www.bikebandit.com/houseofmotorcycles/showschematic/m2877sch3877 Is
the diagraphram part of 13?
Prior to rebuilding, the float level was to the point it was leaking gas
when on the sidestand. The idle screws were set at 2. The bike seemed to
have a little more power, and the idle was rough and speed varied quite a
bit based on how hot the engine was. At the time it did not have the
hesitation.
I currently have the float level at .3" ( float bowl mating surface to the
bottom of the float ). Prior to rebuilding it was .17"
Would raising the float height possibly help? I'm thinking .25? Would doing
this make the bike run richer throughout the throttle range?
.
> I currently curretly have them set at 2 1/2 turns out. �If I open them to 3,
> the idle is rougher and the hesitation is reduce. �At 3, the bike idles at
> ~900 �RPM a couple minutes after it is started and �~1200 when hot.
If you open the idle mixture screws too far, the exhaust note will
become dull and thudding and the engine will run rougher. You may
smell raw gasoline in the exhaust.
I don't happen to know what the recommended number of turns out from
lightly seated is, and, when someone says, "Oh, you should start with
x number of turns," my question is, "But what size are your idle
jets?"
The jet sizing system is based upon a #100 jet having a hole that is
1.00 millimeters. So an idle jet that has a hole that is 0.30
millimeters in diameter is a #30 idle jet.
#30 is a fairly small idle jet, and a #45 idle jet is *huge*. If you
work out the area using the pi X radius squared formula you learned in
high school, you'll see that a #45 idle jet has a much larger area
than a #30 idle jet.
So a carb with a #45 idle jet might need to have the idle mixture
screws set at only 1/8th of a turn out from lightly seated, while the
carb with #30 idle mixture screws might be set at 3.0 turns out.
I looked on the fiche and I didn't see any
> diagrapham.http://www.bikebandit.com/houseofmotorcycles/showschematic/m2877sch3877�Is
> the diagraphram part of 13?
That link didn't work for me.
#13 is probably the vacuum chamber cap. Honda only supplies the
diaphragm as part of the vacuum piston. But there is a company in
England that makes cheaper aftermarket diaphragms.
> I currently have the float level at .3" ( float bowl mating surface to the
> bottom of the float ). Prior to rebuilding it was .17"
You're not measuring the float height correctly. If you have the carbs
upside down on the bench, the floats are upside down too, so the
bottom of the float is on top, and your correct float height is
something around 14 to 17 millimeters.
I dunno what the exact height is, you can google for it.
> Would raising the float height possibly help? �I'm thinking .25?
Yes, but google up the correct float height dimension so you won't
have to guess.
Adjusting the float height so the float valve shuts off later raises
the level of in the float bowl and your engine can more easily suck
gasoline through the idle jets when the level is higher.
Would doing
> this make the bike run richer throughout the throttle range?
Your engine will draw some fuel through the idle jets at all throttle
settings up to wide open, but the engine vacuum decreases as you open
the throttle further.
The most fuel that will flow through the idle jets is at about 1/4
throttle. Fuel flow through the idle jets stops when the throttle is
fully open because there isn't enough engine vacuum to suck gas
through the tiny orifice in the idle jet.
Dennis
the float level 1 mm, and everything is pretty much set by the book
(Clymer's):
Main jet: 115
Slow jet: 40
Float Level: 7.5mm ( just changed to 6.5 mm )
Pilot screw setting: 2 3/4 ( changed to 2 1/2 )
I also synced the carbs while I had the tank off. Its currently 75 deg F
and when the bike is fully warmed up, it's running *perfect*. Until it is
warmed up, it still has a very slight hesitation. I think I'm just going
to leave everything as is for now and spend some time riding and enjoying
the weather before it gets too cold :)
With these carbs, I don't see any midrange adjustment clip? I used to race
dirt bikes and the midrange was adjusted by needle clip position. I am
also confused about the whole purpose of using a diagrapham/piston setup to
open and close the jet needle? All the bikes I had previously it was much
simpler, and the jet needle was moved by throttle position?
Is there a performance improvement by using vacuum vs. throttle position to
control the mix?
.
> Slow jet: 40
> Pilot screw setting: 2 3/4 ( changed to 2 1/2 )
That sounds like a lot of turns on the pilot screws. My Yamaha FZR1000
has #40's and the pilot screws are turned out less than 1/4 of a turn.
> With these carbs, I don't see any midrange adjustment clip?
Nope. If you think you can improve the midrange mixture, run down to
the hardware store and buy a bunch of 1/2 millimeter thick steel
washers with
a 3 millimeter hole, and stack as many as you want underneath the head
of the needle.
If you look at the needle in a street bike carburetor, it doesn't have
much taper, so raising it a millimeter or so isn't going to do much.
The constant vacuum carburetor is designed to keep the Earth's
atmosphere cleaner by taking direct control of the mid range away from
the rider's twitchy wrist.
Back in the 1950's and 1960's, British Amal carburetors were crude,
nasty, dirty-running things that couldn't produce a clean-burning
mixture.
Nothing would flow through the jets until the throttle was opened
quite a bit, and then the engine got a big nasty slug of almost raw
gasoline all at once and bogged down.
Then it would clear out and the engine would run well on large
throttle openings.
Along came Mikuni and they added cross-drilled holes to the idle jet
and the main nozzle (needle jet holder), and the extra air allowed
motorcycle engines to run cleaner on an air-emulsified fuel/air
mixture.
IOW, the air bubbles in the gasoline coming through the idle jet and
main nozzle allowed the gasoline to break up into finer drops.
The British had the classic SU carburetors on their 1960's cars, and
the SU's were even cruder than the Amal motorcycle carbs.
An early SU carburetor had a main jet with a 1/8th inch hole and
vaccum raised and lowered a vacuum piston with a tapered needle. If
the owner wanted to tweak the mixture, he could loose a set screw and
move the needle up into the piston about 1/8th of an inch or he could
screw the main jet up and down in the bottom of the carburetor.
The SU's were too crude and dirty running, so the Bendix diaphragm
carb was invented for British cars in the late 1960.
Keihin produced much improved vacuum piston carbs for Hondas in the
late 1960's, and Mikuni built rubber diaphragm constant vacuum carbs
for Suzuki and Yamaha.
>I am
> also confused about the whole purpose of using a diagrapham/piston setup to
> open and close the jet needle?
Cleaner air. �
Dennis
and how to tweak them :) Greatly appreciated!
SAMMMM
that is to say, when i couldn't get the idle to be right at 1 1/2 turns out,
it could be
adjusted by the float level change.
as to the hesitation, i've found many jap carbs set lean from the factory.
it was always a PIA to get them right, if there was a way.
the fact that it idles better when hot, suggests a lean condition.
are the needles adjustable?
sam
"Dennis" <cctden@sbc_global.net> wrote in message
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