i just ordered a FMF Powercore 4 and a K&N jet kit for my 01 400ex, i want to know what jet i should put in it.

 

you will probly have to rejet it a couple of time before its perfect. start with the next size bigger main jet and then test it out. then try the idle jet and test it out. only change one jet at a time and test it out so you know how the machine is going to run. keep doing this till the machine is running good. when you test it out make sure you take the bike out and run it for awhile so the gas has a chance to fully circulate through the carb. check out

http://www.dansmc.com/carbs2.htm