Small Engine Troubleshooting Problems? Ask a Mechanic Now.
6A.12 page in the above service manual shows the governor for the M20. You have a weight set inside the engine that has to move to controll the top rpm. The governor is driven by the oil pump gear. I think you have a worn governor and the only repair that will do any good is to replace the governor weight and gear.
I see question is still open, did you find problem?