First thing to check is that the governor spring is undamaged and the air vane and its governor linkage are friction-free. You might check the air cleaner: if it is restricted, the mixture will get richer as the speed and load increase, which makes carburetor adjustment difficult. Check that the muffler is not restricted. Aside from that, you could tune the engine - tappet clearance, clean carburetor, replace diaphragm, new spark plug. It is probably one of those things, and other than the tune-up they are cheap and easy to check.
