On the engine itself, find the model number, usually stamped somewhere
on the muffler or close to it. Google the model number for the parts list.
Depending on how handy you are, you can take the carb apart, the bowl
portion, and replace the needle valve seat and the needle valve itself. That
will help fix the mixture problem. The oscillation is caused by mixture
fluctuations. As mentioned, the spring is connected to the governer.
Usually it is just a secondary flap in the air intake. When you remove the
carb assembly, you can see how it works. It is possible to temporarily
bypass it for testing, but monitor the throttle slider as you can overspeed
the engine. I've had multiple small engines with this issue and replacing
the carb parts resolved the oscillation problem. On a few, when I was
lazy I zip tied the governor so I could get stuff done.
----- Original Message ----
From: "email@example.com" <firstname.lastname@example.org>
Sent: Tuesday, August 25, 2009 7:50:15 AM
Subject: [Shop-talk] mower engine issue
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.
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