So, if you pull out the spark plug and turn the engine over slowly using the kickstarter, does it turn over smoothly? No problem with valves hitting the top of the piston? If so, do a compression check to see how much pressure you're building. Did you replace the piston with an OEM replacement? You didn't leave out a base gasket between the cylinder and the case, did you? Did you replace the gaskets with OEM gaskets or at least ones that are OEM spec? Are the camshafts properly timed and valves adjusted correctly? Is the piston installed backward (180 degrees out)?