I just posted about this. Take a flashlight and look into the spark plug hole. Carefully observe the exhaust valve throu two crank rotations, (or one cam rotation). Does the exhaust valve move slightly during the compression stroke? I've seen many older Tecumseh motors where they were impossible to tune. Closer inspection revealed a floating exhaust valve. Pop open your crankcase vent cover and measure the valve clearance through all four engine cycles looking for changes in valve clearance (other than when the valve should be open).