My Craftsman 5.5 hp lawn mower Model No. 917.377700 has begun to run erratic; that is, the engine revs up and down continuously (this model is designed to operate at constant, non-variable RPMs).

I checked the carb choke linkage/springs. They seem to be working like they're supposed to, although the spring on the inner butterfly valve's linkage is very fine/delicate.

I checked the carburetor (Keihin brand--float, jet/nozzle, needle valve, bowl, springs, action--all seem to be working as intended).

Air filter OK. Spark plug OK. Crankcase oil OK.

Fuel delivery system OK.

Any ideas would be welcomed and appreciated.

Thanks!

Calli Pegian, March 2010

Hello, ive just purchased a Hayter Harrier41 that is doing the same thing, reving high & low constant, looks like im gonna need to break the carb open to check that main jet!

Carts, November 2010

Engines hunting is nearly always because you have fuel starvation, i feel it has to be dirt in the main jet.

Husqvarna man, March 2010