I just bought a K&N Air Filter for my completly stock 2004 400EX. I still haven't installed it and I was wondering if I need to get it jetted?

 

Can't understand why everybody asks this question! Of course you do! If you didn't have to rejet, you have achieved nothing, except spending your money! The idea of the free flowing filter is to pass more air, and if you pass more air, you need more fuel, or you will be running lean and will toast your engine.

Throw the K&N on, then try to ride the bike (for a very short time!). It should run poorly without the rejet.

 

Yes, what he said.