OK just thought I'd share a little bit of information with everyone here who rides a 400ex that I did to mine...this stuff works great too!!!
#1.. Get rid of that stock air box. That thing is so restricting its pathetic...I went to JLAtvproducts.com and bought a Filter conversion kit. It completely gets rid of the Airbox and mounts a K&N to the aluminum plate and the intake. I also added a stage 3 Dynojet kit. .. With just this mod done alone it was amazing on how much it opened up the motor...lot more torque

#2.. I took the choke off my 400.....I guess I have the luxury of living on the west coast where the weather doesnt get too cold like in the midwest..but even after it sitting for a few weeks, 3 pumps on the throttle and it fires right up....to cover up the hole on the side ov the carb I used a 5/8 vacum cap with a hose clamp...some people use silicon or jb weld but i like the cap better.

#3.. As of right now I have a stock pipe that has been drilled out..this is great and really does wonders..heres the link

#4.. Since I do a lot of dune riding, I went from the stock 15t front to a 14t....this is like going up 3 on the rear. Front end comes up with ease now in the higher gears...

After doing all of this I can keep up with a 02 400ex with a rev box and a full pipe system....I spent about 100 while "he" spent about 500............have Fun

 

i guess the link didnt work for some reason but here it is again

http://home.earthlink.net/~gellett/exhaust/index.htm