do i need to rejet after a k&n filter with lid still on?

 

Usually if you put a better filter on and a better exhaust you'll need to rejet to compensate for the better airflow through the motor. If you put a better filter on and run the stock exhaust it may not be necessary, as your best benefit will be with a better exhaust and jetting. It may not be worth the effort to rejet unless you do the better exhaust as well.

my 2 pennies worth.

 

Basic theory is that engine doesn't make more power with just more air - needs more fuel, too.