THE POWER STROKE (third cycle)
Both the exhaust valve and the inlet valve remain closed.
Just before the piston reaches the top of its stroke a high voltage is created within the "High Tension Coil" and is fed (via the distributor) to the spark plug. This high voltage jumps the gap between the spark plug contacts and ignites the compressed gasoline / air mixture.
This results in a rapid burning (expansion) of the gasses which forces the piston back down the cylinder bore, and in the process turns the crankshaft. This is Power.....
THE EXHAUST STROKE (forth cycle)
The inlet valve remains closed but the exhaust valve is now opened.
The crankshaft continues to turn, forcing the piston back up the cylinder bore.
Because the exhaust valve is now open the movement of the piston forces the burnt gasses out of the cylinder, through the silencer and out into the atmosphere.
This has now completed the second revolution of the crankshaft. The whole process is now ready to begin again at the induction stroke.
With this short explanation and the aid of these simple sketches I hope you can now grasp some of the important issues taking place here. The sequence (TIMING) of movement of the components is of prime importance. The valves must open and close in the correct order and the spark must arrive at just the right time to ignite the fuel / air mixture. Also the fuel / air mixture from the carburettor must not be too rich (too much fuel) or too weak (too little fuel known as lean).