I have a 02' 400ex that I want to add a few things to. If I were to add a K&N air filter in the stock airbox without any mods to the box and a White brothers E-series silencer, would I etill have to have the bike jetted? Please forgive these newbie questions. Of course, I am a newbie if that helps.

 

No need to apologize, every one was a beginner once.
If you're running the stock main jet, you will almost certainly need to rejet your main jet for the added airflow the pipe will give you. The more discs you run in the WB pipe ,the larger the main jet must be.
As I continually preach to anyone who will listen, the only way to know what's needed in proper air/fuel mix is to do a test ride and plug read. Anything else is pure guess work. With the Keihin carburetor that you have rejetting is a no brainer due to the large main jet access screw in the bottom of the bowl. After you install your filter & pipe, do a proper test ride & plug read to see what your engine wants in terms of fuel .

 

Thank you very much for the help! I greatly appreciatte it!

 

Yes, you would. The size of the jet is 142(if you are running 13 discs.)