250-500cc vulnerable air box

So I'm finding my air box to be prone to catching water. I replaced my crank bearings about 4 mos. ago and then did a 6 speed swap last week and found my crank bearings to be rusty again. I ride in some extremely wet conditions here in the northwest and don't foresee that changing. Anyone noticed this problem & came up with a solution? It's a 09 300 and never sits for more than a week. thanks for any thoughts.

are you using amsoil dominator two cycle oil? You could use Maxima FFT air filter oil on your air filter. Supposed to be water proof. I haven't checked my crank bearings yet.

Amsoil dominator with the first set & then I switched to klotz r50. bel ray filter oil.

I ran a piece of skid plate foam in the bottom of the airbox to keep water from coming up thru the holes and getting on the air filter.

dominator known to cause rusty crank. Don't know about R50.

joedirt hit the nail on the head! water gets directed straight up through bottom of airbox when riding through water-crappy design but easy fix. either use foam to seal bottom of airbox(somewhat porous stuff that will still let water out but prevents it from getting in) or do like I did & clean airbox thoroughly then silicone seal the whole gap then drill a 1/5"(hey see no metric used for you guys) hole in bottom/underside of airbox. work out where lowest point is in airbox when bike upright & on its stand for drainage reasons. I think it angles down to the left side of bike more than the right & rearmost point but youll work it out. makes it a submarine as takes a ages for water to fill it through such a small hole but drains rapidly. just check its clean/clear every now & then. never had a drama with mine since but did drown my bike from riding too fast through shallow water a few times before I did this mod. buggered if I could work out why as I had my carb breathers routed up into airbox(do this also-only the 2 that angle upwards on the carb though) then had a good look at airbox & found the culprit!

wheres that info from shrubitup? im using it at the moment, hope my cranks all good!

Yz250 air box identical regarding drainage outlet. Never a problem.

From amsoil website:

* Not suitable for long-term use in outboard motors as a TC-W3-type oil, although excellent as a race oil for short-term use where motors are periodically inspected.

I take that to mean that it lacks corrosion inhibitors. Interceptor is said to have them tho.

I've been using Dominator for the past 150 or so hrs.Used Motul 800 for the 1st 200.Just split the cases and everything looks new.

It seems to me the water is entering through the top of the box. no deep crossings just lots of shallow water in the trails I ride. the ims tank is not as tight to the upper subframe mount as the original i'm noticing. maybe some sort of foam insert as opposed to the little rubber mudflap.

holes in sideplates/rear fender, tank/seat juction & subframe mount point where rubber mat is(my money on this one) could potentially funnel water onto filter i guess. yeh bits of foam strategically placed should sort it. ive never ridden in real hard rain or very wet bitumen roads so never had this issue but done plenty of wet trail rides with no issues from water/mud

You'd think a well oiled filter would stop water ingress ?
Maybe rig up some sort of deflector ?
Would be pretty hard to stop all water with the amount of water we get around here.

Waterproof doesn't mean that it can keep the water from passing through the filter, it just means that the oil that contacts the water won't turn into an emulsified mess. Anything that could keep water out would severely restrict airflow......