i just replaced the engine off of a kids bike. but the new engine is super slow. i noticed that the muffler was barely pushing out exhaust but there was a bit coming from the manifold and where the stock muffler comes together. also When cleaning the new piston i noticed that the piston itself had a few scratches on the head. the bike starts up pretty well but when you go to drive it there is no pick up and it only hits like 15 max. I tried to remove the little end inside thing from the muffler to make it a straight pipe and see if that helped with performance. It did slightly. Does anyone know what could be wrong. Could timing be to blame? It seems to start easy but i dunno. the bike also seems to have pretty good compression. anyone know whats up?