sound 2 me you may have a leak in the crankcase or possibly in the reeds (if these engines have em) and dont forget the dreaded crank seals.start it and idle it rough,use a pump sprayer to spray water at and along any seals. if you hear a change that will bee your leak,or blow some smoke into carb and look for leaks that way(only works for smokers)you will need to position the piston to cover the ports though.no leaks,goog carb leaves ignition problem.