Anytime there is a decrease in the amount of restriction in the intake and/or exhaust system of an engine, there is a requirement to adjust the amount of fuel as well. In the case of air filters alone, most aftermarket filters reduce the intake restriction and require a larger main jet size. With the engine running at lower rpm and load (throttle positions) there isn't a large enough change with just an air filter to require air screw/pilot/needle jetting adjustments. It's when the engine is under high load/rpm that the air filter allows for better breathing and therefore requires larger main jet sizes. How much larger will depend on whether the air box lid is still used, and the amount of flow increase the aftermarket filter allows. Some vehicles are restricted by the air box much more so than by the filter. In those situations, changine the jetting is generally not necessary. If you don't know if it is necessary or not then it's best to consult a mechanic familiar with your machine.

 

It seems like everyone who adds a K&N Filter re-jets? Is that a routine reqirement? If so, you don't just remove your stock filter and put the new one on?
Thanks

 

Txdoc i got a K&N and for the first couple of months i ran it i didnt rejet it. I did end up gettin a dynojet though(big mistake).
Maybe all you gotta do is up tha fuel mixture to compensate for the added air flow.