On Aug 25, 2009, at 7:50 AM, scott.hall@comcast.net wrote:
>
> so it starts and runs...but it runs with an 'oscillation', I guess
> is how I'd describe it. if you hold the throttle pedal down, it
> won't maintain a steady rpm. up and down at a fairly frequent
> pattern. there's an actuating rod attached to what I think is the
> throttle plate in the carb (there are two butterfly plates in the
> carb--the choke and the plate to which this rod is attached). this
> rod runs under the gas tank and is attached to the motor in some
> fashion (can't seem and wouldn't understand it anyway, so didn't
> remove the tank). this rod is moving back and forth at the same
> periodicity as the engine rpm oscillation. clearly, that's what's
> causing it.
>
> anybody know what that rod is, and what it's doing? some sort of
> constant velocity setup?
That's the governor. It takes engine speed (usually from the air
coming off the cooling fins on the flywheel acting on a vane, or by a
connection into the crankcase) and limits it by being directly
connected to the throttle. The spring is the connection between the
gas pedal, turtle/rabbit handle, etc and the carburetor. It takes the
operator's requested speed and suggests it to the carburetor. When the
engine speeds up the governor acts against the spring to throttle back
to a pre-set maximum (either for lawnmower blade speed federal
maximums, or connecting rod integrity). Do not mess with this if you
do not understand it, and certainly, once you understand it, do not
adjust/defeat the governor to allow a higher speed than they set it up
for ;-)
What I believe is happening is your idle circuit isn't working, still
plugged up. When the engine starts to stall, the governor has less
wind acting on it so the throttle starts to open, activating the main
circuit which runs just fine. Engine speeds up, throttle closes, no
more gas, engine slows down, throttle opens, repeat. You need to clean
out the idle circuit. It's too small to get in there with mechanical
means without scoring it, so you'll need to use chemical means,
soaking the carb for a day or two. A gallon of Berryman's is $30 or
so, but my favorite is Yamaha carb cleaner in the black one liter
bottle if you have a dealer nearby. Follow the directions on the
Yamaha stuff, though I have also used it in teh gas tank and let it
burn through with no ill effects.
jim
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