Carbs operate due to differences between atmosphere and internal air pressures. Changes made to an engine that change that pressure differential will require jet changes to be made to keep the air/fuel ratio correct.
It is outside atmospheric pressure that causes gas to be pushed through the carb jets into the incoming airstream.
Take the airbox mod for example. Removal of or drilling holes in the cover REDUCES the pressure differential..the engine is trying to pull air against less resistance (less obstruction). All other things being equal, the airbox modification will cause the engine to run lean(er) because there is less pressure differential so less gas is pushed thru the carb.
Put a small straw in your mouth..suck as much air as you can, compare that to sucking on a piece of 1" pvc. Your cheeks don't collapse at all with the larger opening compared to the smaller.
Putting on a pipe effects the engine in a couple of ways. It may flow more air from the standpoint of size and volume ability. But, in a two stroke the pressure waves created in the expanding/collapsing cones have more of an effect on engine operation than the 'amount' of air you can get through the pipe based on it's size.
Rule of thumb is adding boyesen reeds or a radvalve will call for a leaner pilot and main. That would mean that modification is changing the airflow in such a way that the pressure differential INCREASES, so you need to make the gas 'harder' to get.
AHA!!..That's two opposing statements! Increased differential with a stock airbox is bad..but increased differential with addition of reeds is good?
Well...that isn't happening on the same side of the carb, 'eh?
Have you been through CDave's carb theory? Check out:
http://justkdx.dirtrider.net
Click on 'tech tips' and 'carb tuning'.
Basically
EVERYTHING/ANYTHING will require rejetting. Temperature, elevation, riding style, induction modification (type of filter, how it's oiled), exhaust modification (type of pipe, S/A or just a silencer with NO arrestor).
Who said this...someone on this forum...'Jet a rider's bike, he can ride for a day. TEACH a rider how to jet.........' well, you get the idea.