250-500cc 2002 WR360 Airbox

I picked up a WR360 a few weeks ago and it's in amazing condition for it's age, still has the original tyres on it. I took it out on the weekend for my first ride off-road and it was abit of a disaster, every time I went through any water the bike would stop and I would have to remove the airfilter and squeeze all the water out of it. I noticed at the bottom of the airbox there is quite a large drain opening in the airbox and it faces rearward. It doesn't appear to have anything covering the opening, I would have thought that it would have some sort of cover that lets water out but stops the rear tyre throwing water into the airbox, is my airbox missing something?

sounds like you could be missing the rear mud guard that goes in front of the rear tire and drops down behind the swingarm. Can''t remember what its called but without that, I'm sure the tire is just throwing water into your airbox.

take a look from the back of your bike and see if you can see where water could get in
take off your seat and do the same
it might be stupid simple such as a missing or broken piece, typically it is a simple thing

I had the seat off when I was getting the water out of the airfilter and that's when I noticed the large drain ( opening ) at the bottom of the airbox ( which I expected to see a drain somewhere at the bottom ). It also has the section of rear mudguard fitted that comes down to the swingarm but it's not wide enough to stop all the water being thrown forward. I think it must be missing some sort of cover over the drain hole, I don't imagine they would build a bike with a large opening facing rearward that water can quite easily enter. My brothers Gas Gas has a rubber flap over the drain hole in the airbox.