weedeater carburetor: how does it work?
George
looking at a weedwhacker, which is a little different. It has a diaphram that
controls the needle valve, which seems clear enough. But, I don't see how the
diaphram is operated.
The fuel chamber has a vacuum line coming from the primer bulb, which works OK.
But, after the engine is running, I don't see where the vacuum comes from. The
primer also pressurizes the fuel tank.
Anyway, if someone has a pointer to a picture of this sort of carb, I'd
appreciate seeing it.
Thanks,
George
Kevin
"George" <_g_bnotreallym...@verizon.net> wrote in message
news:gjbv62lkkpn8n1qbm...@4ax.com...
Two cycle carburetors work on pressure and vacuum pulse developed in the
engine crankcase. The diaphragm in the carb moves back and forth because
these pulses are ported from the engine into the carb. Simple flap type
check valves control the channeling of these pulses.
The following links relate to chain saws, but the carburetors are very
similar in operation. You might find them helpful
http://www.madsens1.com/engine.htm
http://www.madsens1.com/sawtune.htm
--
Kevin Mouton
Automotive Technology Instructor
"If women don't find you handsome, they should at least find you handy"
Red Green
thetoolman
The type of carbs that are on weedeaters, chainsaws are operated by the
pressure/vacuume pulses from the crankcase of the engine, which moves
the diaphagm back and forth raising the the needle valve off it's seat.
This disphragm also serves as the fuelpump too. If you look very close
you will see a small passage that lines up with the plastic carb mount
this is how the pulses are transfered from the engine to the carb. As
the piston moves up and uncovers the intake port, fuel is pulled into
the crankcase. The primer bulb "pulls" gas out of the tank and pushes
it to the carb. The engine will not run if there is pressure in the
tank, you will see a oneway valve in the gas cap to let air in but not
let the gas leak out. Most of the time the diaphragm gets stretched out
and will not lift the needle off the seat, causing a hard start or a no
start condition. Most Outdoor Power equipment store will carry a
rebuild kit for the carbs but you will need to get the numbers off the
carb. Kit comes with the diaphragm, needle valve, needle lever and
gaskets.
HTH, Rick
thetoolman
for acting as a fuelpump with two flaps that act as valves AND the
other for the needle valve. Boy! it helps if I look at the manual first
before I post....My Bad!!!
Rick
five...@webtv.net
>mho
>vƒe
>drive 10% less, help create a gas glut
George
What confuses me is ..
1. What I guess is the fuel pump diaphram (with the two flaps) is pretty stiff,
and pretty well captured under a gasket and cover. That actually flexes up and
down?
2. AFAICT, the needle valve diaphram is only connected to the primer bulb. The
crankcase port seems only to go to the pump - I don't see a passage connecting
connecting it to the needle valve diaphram. I think you need some sort of
vacuum there, to operate the needle valve, but I don't see where it comes from.
George
Kevin
There are internal passages drilled through the body of the carb that you
can not see, at least not with out total disassembly, including removing the
welsh plugs. Even then, you might need a magnifying glass to see the tiny
holes. If you are all that concerned about the design and engineering of
the carb, then you should invest in a good book on the theory and operation
of two cycle carburetion. Try Amazon.com. If all you want to do is get it
running, then an owners manual for your equipment is the best purchase (
again Amazon.com) or just follow the help and suggestions already posted
here in this group.
Good luck,
jim
George wrote:
>
> 2. AFAICT, the needle valve diaphram is only connected to the primer bulb. The
> crankcase port seems only to go to the pump - I don't see a passage connecting
> connecting it to the needle valve diaphram. I think you need some sort of
> vacuum there, to operate the needle valve, but I don't see where it comes from.
The needle valve, diaphragm and spring arrangement is a pressure
regulator. The pump feeds gas thru the needle. As the pressure under the
diaphragm increases it pushes the needle onto its seat (reducing the
flow). This maintains a pressure equilibrium. From there the fuel flows
into the air intake (how much depends on throttle position). As fuel
leaves the chamber the pressure is reduced and the diaphragm moves in
the direction that lifts the needle off the seat. So the only vacuum
involved is the vacuum in the air intake that draws the fuel thru the
passages from the chamber under the diaphragm. When the engine isn't
running there is no fuel being drawn from the chamber (and no fuel
pressure being pumped in) and the spring should be just barely pressing
the needle against the seat and closing off the flow.
-jim
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thetoolman
1) I agree with Kevins last post.
2) Yes, the diaphram does flex back and forth....diaphram should be
soft and flexable. The diaphram gets stiff with age. The big problem
with these carbs is that people let the fuel set in the tank for too
long and it GUNKS up the carb. I pour out the fuel of my chainsaw and
start it up and let idle till the engine quits. I only make enough fuel
mix for the job, one gal. at a time
Rick
Kevin
"thetoolman" <maxx...@yahoo.com> wrote in message
news:1148219298....@j73g2000cwa.googlegroups.com...
Hey Rick,
I used to drain my power equipment for storage until I discovered "Stabil".
Have been using it for many years now and never have a problem with fuel
oxidation or varnish build up, even when the tool sits for over a year. My
generator has gone as long as 3 years without use and started right up with
no fuel problems. Try it, you might like it. Now, I always store my
equipment with full fuel tanks and Stabil additive. I find it helps prevent
moisture condensation that you get if you store the tank empty.
thetoolman
keep the gas in my generator all the time too, although I run it at
least once a month under load so I know it will work when I need it.
Rick
Scott Dorsey
Stabil isn't a miracle... but it will let you keep fuel in your lawnmower
over the winter without any problem.
If you put Stabil in a generator, then wait five years before using it,
you will have problems. I'm not mentioning ANY NAMES but I have a couple
customers who have done this and wondered why they weren't getting good
power....
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."
Kevin
"Scott Dorsey" <klu...@panix.com> wrote in message
news:e4sgfo$fh1$1...@panix2.panix.com...
Yeah, 5 years would be a stretch. It is really best for winter storage, but
using it sure helps keep the fuel in good condition a lot longer than with
out it. At least that has been my experience.