Have you explored the crankcase ventilation system? There's a good bit of information on www.ch250.net
in the tech tips. If the ventilation system is plugged you will get a lot of pressure just from the single piston going up and down, and that pressure has to go somewhere. For some reason people seem to think that ripping off the ventilation system will released massive amounts of undiscovered horsepower, and so they do get vandalized from time to time. And, the engineering Honda used in the design was not great either.