you can hear the piston moving and there's no resistance on the tire? You, my friend, have a major compression issue then, just check to make sure all the parts and gaskets are fitted tightly, and all the head screws are tight. If you have a spark, and there's a good amount of resistance, I might have suggested an air leak on the intake manifold (either at the gasket end or the carb end).
I've been having compression issues too, though not to your extent, apparently these motors should have around 100psi of compression from the factory, I just tested mine at 80psi. I thought my issue was bad port timing, turns out my ports arent even big enough, just weak compression (after porting, with a reed valve, shaved head, stuffed crank, and corrected transfers, these motors can hit 120-140psi), considering my motor feels about 2/3 as powerful as stock, I'd say an extra 20-40psi could make a rather substantial gain in power.