I never did try to figure out what the problem was. Just junked the engine when I saw the damage to the top of the piston. Could have been a spark plug problem,or maybe even a ring gap (too large or small) problem. Don't know, but I bet someone on here may have a little more imput for you so it doesn't happen to you again. Just so you know, the 4.0 head will fit the earlier carbureted models with a major gain in hp, if you go the short block route and find an older one. Who knows, they may even be less money. Just do a little research on it to make sure you mix and match correctly.