Hello, my China-flatter with GY-6 engine 50cc pops out of the exhaust when I go off the gas, about 3 sec after I take the gas away, there is a misignition in the exhaust with a loud bang ! Otherwise it runs quite well ( all throttles away, about 70 km/h according to Tacho = nearly 60 real km/h. ) What can this be about ? Valve play, spark plug or carburettor setting ?