I am running a stage 2 jet kit in my 400ex. Part of this is to take the airbox lid off. I was on a pokerrun yesterday and was cautious around water but still ended up with the water almost up to the air filter when I got home. I was wondering if anyone leaves the plug out of the bottom of their airbox.

 

I took the lid off my wife's 465 and also removed the bottom plug and snorkel. We live in the desert so we don't see much water. A water repellant prefilter will help too.

 

You might try cutting a large hole near the back of the lid insted of removing it. That will help keep the water out and still give you extra air flow. What does come in just falls past the filter. I also recomend a prefilter to help keep the water out.
You can run with out the plug but I'd try putting a small hole in it insted, that should let water run out and keep dirt from being sucked in.

 

Best way, cut a 3" hole towards the back of your filter. Put a speaker port in it. Silicone around the hole. This will direct the dirt/water to the bottom of the box, not directly hitting the filter. Then take some old cushion foam and cut it to fit inside the hole when it's real wet out. I also added a screen to the bottom of the tube to catch the big stuff and keep the foam in place.

Or, get the airbox cover from K & N. It's a waterproof(sortakinda anyway) mesh material. It velcros to the sides of the airbox. But it will lift in some areas over time. I bought another lid ( $8.00) and removed alot of material on the top. I left a 4" strip of plastic in the middle to cover the filter, and attach this over the K&N airbox cover. This does a good job, but I don't trust it in the real wet conditions.

I can go back and forth between lids without too much trouble. I run alittle rich with the tube style lid, but it's okay.

I'd leave the drain plug in it. It will suck up water from the bottom. If you feel you have water in there, just pull the plug on a pit stop or when you come to a major traffic jam in the middle of a race.

 

on my 300 ex I cut a small pice of radiator hose and put a pcv valve, that had a barbed hose fiting on each end, on the airbox drain it works great. if you have a K&N air filter put an outerwears pre filter on it it helps repel spashed up water. I have gone through water over my exhaust with this settup and my quad has never gargled, spit, or sputer. it onlycost about $4

 

I bought the outerwears lid cover and also noticed improved power througout . Get rid of the lid the outerweardoes a good job just keep your filter clean.