i had a similar problem in my CEN... turned out to be the piston sleeve in the cylinder... obviously during manufacture they didn't grind back the burrs formed when drilling the transfer ports... if the top of your piston seizes and you take the top of your engine and can see half of the exhaust port, this could be the problem, the piston catches on the top edge of the transfer port. all i did tou fix it was to get some reasonably fine sandpaper and sand off the inside edge of the ports in the piston sleeve.. runs like a dream now