i have a 2001 400ex with mods that include fmf powerbomb header fmf powercore 4 pipe dynojet kit k&n filter with outerwear edlebrock carb hindson raceing clutch kit and various styling upgrades such as race cut front plastics maxxis razr tires lonestar twin row axle carrier. my question is i have heard alot of people are running there 400's without an airbox lid would this really do anything for me given the stuff i already have done?

 

It should, especially combined with those other mods. Going to deliver a lot more air than that little snorkle opening is giving you now. You will need to update your jetting....

I highly recommend the EHS Racing airbox lid (covered with Outerwears), rather than just running wide open.

 

Welcome to forums Ride400EX.
It will help alot. As reconranger said given your setup you will benifit more than stock. You will have to adjust you jetting to compensate for the added air intake. I think you'll be surprised.

 

Instead of removing the lid alltogether, although this works great with rejetting, try taking it off, flipping the two back clips up and into the box and then reinstalling the lid using the front and side clips only. This cracks open the back of the box and greatly improves air flow. You will need to rejet if you do this too.
I did this with my wife's 400ex when I installed an HMF slip-on and it made a big difference in terms of power.

 

what do you think about the k&n air box lid?