The mixture is so rich that it only combusts every fourth stroke -- or every other time the piston compresses the mixture and reaches TDC (like on a 4-stroke engine). It should combust every second stroke -- or every time the piston compresses the mixture and reaches TDC. There is not enough air in the mixture, so the engine needs the extra pump to bring fresh mix into the burn chamber.
You know your engine is four stroking when you near top speed the engine goes from a smooth high pitched ziiiiiiiiinnnggggg tone to a lower pitch rougher tone, and the engine acts like it is choking, like it would like to go faster, and sometimes it will go faster if you close the throttle a little bit.
To fix a four stroking problem:
- Clean your air filter to remove any blockage - this may solve your problem.
- If you've cleaned your air filter, and you're still four stroking, you will need to put a smaller main jet in your carburetor.
Article adapted from this discussion about four stroking in the MA repair forum.