I recently bought an 02 400ex, and has the following mods: different cam, different piston, K&N airfilter, different jets, random exhaust! My issue is i want it to go faster, but first i have to get it running right! When i got it, it ran awesome the first weekend i got it. I took it out about a week later and it started missing! Now the more i play with it, it seems like its only when it gets hot(it misses real bad then kills)! I had the coil checked and also swaped the CDI box with one off another 400 and still does it! I'm at a loss! Please help before i bring it to a shop and dump money into diagnostic fees!!

Damn!!! Nobody??

I can appreciate what your going through...I had simular problems with my Raptor a couple of years ago when I decided to "simply add a jet kit"...big mistake on my behalf.

I ended up taking it to a good mechanic friend of mine who knows "intake" stuff and he was able to get it "purring" in about an hour or so...something to do with adjusting the High/Low speed needles and moving the clip up/down.

Long story short, you can do more damage than good if you do not know how to work on intake problems...for me I spent the $$$ and got it fixed.

I would suggest the same for you...that's my $0.02, spend it anyway you want.

Good luck!

The kid i got it from said he had it done by a shop, but who knows! I'm going to have to bring it in b/c i'm likely to never find the problem myself! Thanks

You mentioned all those modifications, but no where did you say anything about a jet kit. If you haven't re-jet it, I'd start there.