No valves 2 cylcle maybe the way the fuel was delivered through the intake and not proper air to fuel mixture with the bolt protruding in the hole causing tuberlance in the mixture. This is why racers port polish the heads and intake runners to increase fuel flow. Lets not get to deep but happy to hear you've solved the problem.Hay I have an idea ! put the original bolt back in and see if it backfires again???? Lets test the out come
