What happens if You Remove the air box lid on the 400ex, is thier any power gain

 

yes there is a definate power gain. i ride my friends all the time and he took his off. now i can't stand it with it on. it just seems wimpy with it on. it pulls harder with it off. we haven't rejetted yet either.

 

I don't know about the EX, but let me tell you that on a banshee, it's not a fun stuff if you don't rejet. I tried...and it was so lean ir would cut at high rpm. Now I just changed my carbs (to 2 x 34mm ) and I took the airbox lid off. Coupled with a K&N, I hope it will rock.
I know that the best european enduro rider "rejets" his 426 WR-F by putting the airbox lid on or off. 4 strokes seem to be less sensitive to this.