Check your carb and head for leaks. Also go over the engine block and look very carefully for cracks. I was having similar problems with my engine and it turned out that the carb was leaking air due to some modifications by the previous owner. Also, have you tried changing your tune a little bit? It could just be a mix problem. I would also make sure that your piston sleeve doesn't look scored at all. And if all else fails call Cen. They could give you some other ideas or they may want to take a look at your engine.