Need Some Help

Ok heres the thing, my 2001 400ex is sputtering at 3/4 to full throttle. It has great throttle response up until that point. I'm running a k&n jet kit iwth filter and no air box lid. Also a WB e-series slip on with 12 discs. I was originally running a 165 main and thought maybe it was a little rich so i switched to a 160 but still sputters at high rpm's. What could be causing this? Any suggestions would be awesome. Thanks in advance!!!

Matt

[skemp]

Dynojet kit suggests a 170 main (from THEIR selection; Dynojet jets are sized differently) using a K&N filter and no airbox lid with an aftermarket pipe. Try that I guess, but from what you have said, it sounds like you are running rich. If it is more of a skipping instead of a sputtering, you are most likely lean. Try the 170, and if it keeps doing it, or gets worse, try a 155 or whatever is in the kit that is less than the 160. It is all about trial and error.