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Re: [Shop-talk] mower engine issue

To: Shop Talk <shop-talk@Autox.Team.Net>
Subject: Re: [Shop-talk] mower engine issue
From: Jim Franklin <>
Date: Tue, 25 Aug 2009 08:29:41 -0400
On Aug 25, 2009, at 7:50 AM, 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.

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