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This is the 7.5 HP Briggs engine on a Craftsman snowblower. The choke switch is a 5 position switch (far left choke, far right to run). It starts easy and runs smoothest with the in the center position. However, if I leave it in this center position, the engine almosts stalls when I engage the auger, but recovers. When I move it all the way right to the "run" position, is oscillates. In the run position, it does not stall or want to stall under load, and does run. However, it oscillates up and down.


Gas is always fresh and always has Sta-bil in it.
I have attached a video link of the oscillation: Video91.mp4 video by yankees_1996 - Photobucket
 

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I had a similiar problem with a lawnmower, it turned out to be a clogged air filter. In your case, you may have some dirt in your carb, restriciting gas flow. Either fuel or air is restricted, check the needle on the carb.
 

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I've had over 15 machines come in this year with similar issues. In every case it has been some kind of air/fuel mixture problem. Blocked carbs to water in the gas. I even had one machine missing the exhaust gasket from the factory and another one where the gasket was leaking from original install.
Air leaks from either the intake or exhaust will render the machine with problems.
 

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How low is the idle set? If it is to low the engine will bounce on the governor, as it will go below the design RPM I seen this quite a few times. Before you tear into a machine that runs fine under load tweak the idle up a little good chance the oscillation will go away.
 
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