My guess is you have the rotor timing 180 degrees out of phase. The port in the rotor should be starting to open as the piston goes up towards the compression stroke. That results in a low pressure in the crankcase causing the air fuel charge to rush into the crankcase through the carb. If it's 180 out, the rotor port will close as the piston goes up and opens when the piston comes down. As the piston comes down it pressurizes the crankcase and blows the fuel charge out of the carb.