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Model 311777 ...Starts easley, idles right,runs great up to about 70 to 80% of max rpm then begins to misfire sometime backfiring thru muffler ..new carburetor,new spark plug,, valves adjusted to .004 and .006... Any suggestions??

In this case the first thing I would check is to see if the the engine is running on both cylinders. Remove one plug wire and see if the engine starts. Repeat one the second wire. If the engine starts only on one cylinder then the other side is not firing. Second would be to see if the flywheel key is starting to shear.

Rivets said:

In this case the first thing I would check is to see if the the engine is running on both cylinders. Remove one plug wire and see if the engine starts. Repeat one the second wire. If the engine starts only on one cylinder then the other side is not firing. Second would be to see if the flywheel key is starting to shear.

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Rivets said:

In this case the first thing I would check is to see if the the engine is running on both cylinders. Remove one plug wire and see if the engine starts. Repeat one the second wire. If the engine starts only on one cylinder then the other side is not firing. Second would be to see if the flywheel key is starting to shear.

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Thanks for the reply however as stated this is a model 311 777 one cylinder engine I did check flywheel shear pin and it is in good shape

4man said:

Thanks for the reply however as stated this is a model 311 777 one cylinder engine I did check flywheel shear pin and it is in good shape

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Somebody may have to correct me on this but Perhaps your spark is cutting out at higher rpms. Maybe your timing is wrong.
Or misfiring.

Does it surge at all at higher RPMS?

Have you checked for any air leak when the engine gets warm? You may have a gasket that is separating, or a plastic intake that has a hair line crack. Take some carb cleaner and spray around the gaskets and carb, if the RPMs change you have found a problem. How did you check the flywheel key? Without removing the flywheel,it may look good, but even a small change in size will affect timing.