[Frank v R]

have you done a compression test and what type of fuel are you running down there do you still have leaded fuel? sounds like your engine is going away, if you are running unleaded fuel have you been using lead additive? if it is your engine and it comes out make sure you have hardened exhaust valve seats installed ,first thing try adjusting your valves and see what happens, also do you have ethanol in your fuel down there , this stuff is a even bigger problem for the old engines and fuel systems, hope this helps,
regards Frank