See it run : slow fast topspeed


Or like Paul Angyal (paltron@interlog.com) uses to say :
Stroke 1: starting from the BDC (bottom dead center) the piston is moving upward and opens the intake port sucking in fresh mixture into the crankcase, then pushes it up through the transfer port to the ignition chamber and compresses it there. A few degrees before TDC (top dead center) the spark plug ignites and combustion follows.
Stroke 2: the explosion pushes the piston downward providing torque (this is the actual labour phase). As it passes the exhaust port the burnt gases exit there; immediately after, the transfer port opens and the fresh mixture in the crankcase below goes to the compression chamber.