I have a 00 400ex I have cut holes in the airbox lid and am running an outerwears on a K&N filter I also put the white bros. rev kit. The biggest gain i have felt yet is the holes in the air box the K&N didnt feel that noticeable and the rev. kit seems to allow you to turn the bike way out of the power range cant say it helped much if any at all maybe on a motor that has been modified and maybe a larger cam it may help more but for stock its a waste of money. I put 8 7/8 of an inch holes in the rear portion of the lid so the filter is somewhat protected I also have purchased an extra airbox lid and have swapted them back and forth it definately noticeable with the holes in it. Now the big question should i jump the main jet any and where does the air mixture screw come set from the factory Thank you for any advice you may have.

 

I would recommend the 155 main jet,you need to Buy the Dynojet kit $45. The needle has a performance taper to it. The air/fuel stock is 3/3.5 turns out. Put your stock CDI back on. Read my posts about Caution about WB CDI. The 400 makes max power below 7500 well below the 9200 redline. Also causes hard starting. Both boxes are at full advance by 3000 rpm anyway.