I was thinking of just hooking up a K&N filter right to the carb, and getting rid of the airbox. My buddy did it on his Banshee, can I do it on my 400EX.

 

Unless there is the possibility of encountering water or mud, there should be no problem. Of course you'll have to upgrade your jetting to compensate for the increased air flow, particularly your main jet.

 

I'm not aware of any filters made to clamp onto the carb on a 400EX. I don't know how the Banshee's do it, but there's no room for the filter to clear the rear shock. I fabricated an adapter out of ABS plastic (hard plastic sewer hose) by glueing two pipes together. One fits inside the stock snorkle, the other is of the same diameter as the filter ID. Then I fab'ed a bracket to bolt this unit to the frame so it wouldn't wiggle off the carb.

 

i just did the same thing to my 400ex. the hose the connects the airbox the the carb is removible. connect t one end to the carb and the other to the filter and rejet and your set

 

You can buy a UNI filter, just look for one with 2 1/8 or 2 1/4 " opening, it will attach to the carb and it will fit. I just bought one. It is about 3" wide by 3" long. You have to smash it a little with the subframe, but it works. Now I just need to figure out how to rejet.....