I think I must have done something wrong when I installed the piston. I put the engine back on and when I looked in the exhaust port and pedalled the piston doesn't move. I have to turn the flywheel by hand to get the piston to go up and down, and when it reaches TDC the flywheel locks and the piston stops. Then I have to turn it the other way to get the piston to go back down. Please help!!!!!