I have 2000 400EX with a FMF Power Bomb header and a Power Core IV2 muffler. I am running a 155 main jet with a 40 pilot jet. I just purchased a K&N Power Kit. I was wondering if anyone has installed one and what combination they ended up with? Thanks!

 

if you remove the airbox, you ought to be able to at least run a 165 main jet with that header and pipe...

do plug tests to get the right size... but a 165 would be a good place to start.