Clark Mason - Crankcase Breather Tube


I read your post on 7/9/00 about removing the stock breather hose, replacing it with 1/2" braded PVC, running it up behind the number plate and adding a UNI-Filiter at the end. Is this still working OK for you? Have you had any problems? I had incident last weekend where I got water in my oil through the vent tube...stalled in 2 1/2 feet of water. It literally sucked!


Dan in Washington

2000 WR400F

I have been running this setup for the last 6-7 months with absolutely no problems. Its cheap insurance to prevent ingesting water or dust when starting the bike in these conditions. Its also nice not having to deal with the little oil drip on the garage floor following each ride.


The water thing hapened to me as well. My bike actually locked up with a screech days later. It did this 4-5x. I tore my top end down and found some scoring on the exhaust side of my cylinder.

I brought it to the shop to see about honing. The mechanic (I use the term loosely) said it looked more like dirt or foreign material, as we say in the nuclear industry.

I contacted Eric Gorr about his 420 kit. He said his history on this bike is the cylinders do get damaged from crap getting sucked into the top end. I have not done the Clark thing, yet, butdo plan on it as well.

Thanks Scott and Kevin for your response. I ran a new breather hose up the frame and terminating it behind the number plate. It worked like a champ on today's ride.


Dan in Washington

2000 WR400F

Another option is to reroute the existing hose up under the seat to the airbox, again with the piece of foam zip tied onto it to keep the grunge out. I've run that way all last year.

I'm still running the same parts with the crankcase breather up behind my head light and have experienced zero porblems in the since I installed the set up last year. I provided parts to Scott H and as stated above its working fine on his 98 WR, mine is a 99.


I've noticed old XR's run a hose down below the engine with a split nipple fitting on the end to help prevent dirt/water entry. They then also have a tee in this hose with another short hose to the airbox.

That way the lower hose cannot suck up water, as it would find it easier to suck air through the hose from the airbox.

Of course, if you lay the bike on its side drowing both the lower hose and also filling the air box with water, then it might suck water. But I reckon the carby would win the suction contest until the hot bore filled with nice cold water....ouch.

With a hose running up higher than the engine you will have to be careful if you drop the bike in water. When you return it to thr correct "shiny side up" position, anything in the hose will drain straight into the head.

Good point on the severe flooding followed by tipping the bike rightside up again letting the water flow down into the head. If the hose was on the steering head the same thing would happen.

If you submerged the bike enough to cover the seat, water would have flooded through the carb already. At least presumably that water would only pass through the cylinder and not get into the crank, cams and tranny. A low-exit down hose would fill if the bike went down on its side (all the way flat) and water would flow in as it lay down. There doesn't seem to be any good hose position for total bike drowning. It would seem to me that a one-way blow off valve, like a PCV valve, is the only solution. One would just have to hope that any high vaccum situations didn't suck crap into the shaft seals etc. I think I might go for the PCV valve and run the hose downward again, except for the question: is there enough sucking going on during a kick start to pull junk all the way up the hose (appears to be from previous posted stories) and if so, the crankcase/tranny might experience a lot more vaccum than it was designed for, overpowering other seals and sucking in crap. Catch-22. (?)

Idea: run another PCV valve with a medium-strong closing spring that will let excessive vaccum overpower it and suck in filtered air (or filtered water!!). Presumably, the bike has died by this time being completely submerged on its side, so the valve doesn't experience engine-generated pressure pulsations and instead just stops a foot or two of water head pressure from flowing in (27.7" of water = 1 psi valve opening resistance required). I think I have just found my new venting arrangement for deep water.

Just thinking about a total trail flooding of a shiny new expensive hi-tech 4 stroke engine with muddy water makes you want to carry some jugs for a quick oil change - 1 1/2 liters is almost carryable without too much grief, if you know you're heading far into submarine country. (vs a $$$ engine rebuild)

Are there any photos of these air breather setups available, or could someone take some?

The descriptions you guys are providing are pretty good, but a picture is worth a 1000 words ...

There is normally a little oil that leaks out of the breather tube. With the air breather/filter behind the number plate oil can't drain out of this tube. Are there any problems associated with this?


Just what I was thinking Darb. I took my tank off but was unsure how to route the breather hose to the number plate. A few pictures would help me out alot.

The oil draining out is sort of a mistake as one would want to lose zero oil. It's just splatter that bounced up off the cams etc and slowly dripped down the inside wall of the tube. If your tube is clean inside (i.e. clean it and then put a dust filter on the end) there's nothing wrong with the oil splatters running back into the engine.

To route the hose to the air filter, just bend it left off the head (clutch lever side) to do a U turn, run it mostly level, straight back, past the clutch lever side of the carb, whereupon it just naturally ends up popping up to the to edge of the airbox. Just slip it under the frame crossmember. Then I ziptied some foam on it to stop dust from getting into it. I didn't even have to take off the gas tank to do it, just the seat. Sorry, no pics.

Just remember not to take it for granted that the engines ok after a total submersion where you dump the bike onto its side under water.

I'm kind of new to this bike. I have a question. Which hose is the breather hose?



The bigger of the two black hoses coming from the engine head cover.

Thanks Hick! One other question if you don't mind? I read alot about the octopus,what is it? :)


Originally posted by Dens Dirt:

Thanks Hick! One other question if you don't mind? I read alot about the octopus,what is it? :)


I dunno, I have a YZ. :D

“Octopus” refers loosely to the assortment of (emissions related) hoses snarled around the carb on a WR. For whatever reason this only came on ’98 and ’99 WRs.

I think.

Look in the tech section of WR thumpertalk and you will find step by step instructions on its removal and recommended resultant jetting changes. Authored by Clark Mason I believe.

I've been running an upside-down PCV valve in the breather hose for two years. I inadvertently did a 100% submersion test last summer when I killed it crossing a 3-4' deep water hole, and still got no water in the oil. The airbox, carb, & exhaust had about 3 gallons of water in them. Here's my posting from last May:

I used an automotive PCV valve, installed back-to-front. It's mounted in the hose, about 2" from the end. The internal spring pressure is very light (actually the valve is slightly open when inverted), and allows the blowby to escape without restriction. However, a vacuum on the engine side immediately shuts the valve, preventing anything from entering the engine through the valve. I used a Deutch PCV131, but I'm sure others with very light spring pressure would work as well.

Steve - is your PCV in the low "stock" position - or did you move it up behind the light or in the airbox?

Hi Michael,

Yes, the hose is still in the stock position, and it still drips oil as before...

today i put a t-peice in the breather hose, just after it bends over to run down the frame,,, to this i added a length of tube and ran it along the frame top tube and just inside the top of the airbox,

hopefully this will prevent any water beinmg sucked up thro the bottom of the breather pipe,,and any water that gets in the new pipe will just run down the breather pipe when it gets to the t-peice

Sandracer - I've done exactly the same & also added a small filter on the end (pneumatic filter) to stop any dirt etc. I think its the best solution in putting the T piece in instead of re-routing the existing pipe.

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