Piston Porting Explained

Piston porting refers to a method of fuel mixture induction on two-stroke engines. As the piston travels upward to compress the mixture in the combustion chamber, creating a partial vacuum in the crank area, the base of the piston skirt travels above a port cut into the cylinder wall which allows the vacuum to draw fresh fuel and air in from the carburetor. When the mixture in the combustion chamber is ignited, the piston is forced back down closing off the inlet port in the cylinder. The gasses trapped in the crank area are now compressed by the piston until the crown travels below one (or two - depending on the model) more transfer port cut in the cylinder wall. The compressed gasses escape into the combustion chamber forcing the remaining burnt gasses out the exhaust port (also cut in the cylinder wall). And the cycle begins all over again as the rotating crank forces the piston to travel upward.

Although piston porting is the oldest and least advanced form of two-stroke induction, it has in it's favor extreme simplicity. Also, as far as Vespas are concerned, the days of piston porting coincide with the high point of esthetic design at Piaggio. By 1990's mechanical standards, these bikes are very primitive and inferior, but in terms of esthetic design, well...