Cleaning the exhaust port
by Had Robinson
After many hours the inside of the exhaust port and the manifold can have substantial deposits of carbon and/or lead phosphate/monoxide. These deposits can restrict the movement of exhaust gases (which also carry unburned fuel for recycling) and should be removed.
The easiest way to do clean this area is with an ordinary pipe fitting brush that plumbers use to clean the insides of copper fittings. These brushes are available in hardware stores in various sizes in the U.S. The most common are 1/2", 3/4", and 1". The 3/4" size generally works on 80cc and 100cc engines. The 1" size works on the larger engines.
Remove the exhaust system and turn the the crankshaft so that the piston closes the port. If this is not done, debris will get inside the cylinder and, worse, move into the cylinder intake ports.