This makes sense, as the exhaust and stock airbox restrict the engine. In most cases the preinstalled main jet is a 55.
Let’s apply this formula to an example: On a stock scooter you often have a 12mm carburetor installed, along with a restricted stock exhaust and a bulky stock airbox. A perfect engine is determined by perfectly matching components and ideal condition (sea level, perfect weather, etc.) When using a racing exhaust and an open air filter (along with a good reed valve, etc.) you will notice that this formula will give you pretty much perfect results for any carburetor based engine (both 2-stroke and 4-stroke). The idea behind this formula is to give you the ideal main jet size for a theoretical perfect engine.
