breather pipe

Ive heard storys about the breather pipe coming from the head sucking up water when in deep puddles etc and wrecking the engine.

is this true and how do you all get round it?

I have also heard this. That is why I rerouted my breather tube to the air box. Not behind the filter, but I figure if it sucks water from that height, I am screwed anyway.


Doesn't your breather hose blow a little bit of oil? It seems like running it into your airbox would make a mess. My 2cents.



you "can" suck water up it *IF* the bike is started with the hose in a puddle. once running it will not pull water up.


what kind of a fitting did you use to connect the tube to the airbox?

where did you get it?

I've heard stories about people installing a

PCV valve on the hose and this is supposed to

assist in creating a vaccuum in the crankcase and will cause the piston to pump less air letting it move through the stroke much easier thus increasing the HP. MxTuner was

talking about this on the yzwr400 list. Here is an excerpt:

The problem with the stock set up is that if you stall it in a creek deep enough to cover the end of the hose, when you kick it over, it can suck

426ccs of water into the crank case. I'm a bit nervous about that.

I've seen a few different set ups on a variety of machines. I think the best set up for horsepower is routing the hose to a fitting on the

exhaust pipe. But the length of the hose is critical for getting the maximum benefit horsepower-wise. I talked with a local bike drag racer who did

some fairly thorough research on this. I had heard about it first years ago in my VW Bug hot rodding days.

I've put a filter (Uni makes a small one that has a spigot that fits the end of the breather hose for about $12) in the end of the hose on a few

bikes. These, I normally run into the top of the airbox just to keep it out of harms way. These eventually get an oily build up on them (and yes,

dirt sticks to it just like the air filter).

I've heard the Euro versions of the WR run the breather into the air boot in the fitting that is at the top of the stock air boot. The Euro bikes have

this fitting that a 90 degree elbow fits into and the hose fits on that. When the original owner of my bike brought it to me for it's initial set up, I

bought an elbow fitting, cut the rubber casting in the stock boot and ran the stock breather hose to it unfiltered. Never had even the slightest hint

of a problem with it. No unusual oily accumulations, no running problems, no oil consumption issues, etc, etc. In fact, this particular WR 400

would out pull even modified YZ or WR's by a big margin in any horsepower situation. Whether it was due to the breather routing, I really have

no idea. But I'm quite certain it didn't have an adverse affect on it. When I pulled it down to install the big piston (after two full hard seasons of

enduros, hare scrambles and much practice), everything still measured well within specs.

Just my findings with what I tried. Hope this helps.

MX Tuner

Just a bit of advise, that hose pumps alot of bad air (low on oxygen content) out of the crankcase. I wouldn't route it into the airbox. Also I was told that if that hose gets plugged or pinched that the oil pump will not work due to pressure buildup in the cases.

F-Pilot, That has to be one of the most ridiculous comments I have read on this forum. About a lot of "bad air" coming out of the breather hose. The air that is being vented out of the crankcase is the same as the air that is in the crankcase, so by venting into the airboot, essentially you are not doing much to disturb anything. Every car that I have ever worked on, has the same exact set-up with a breather from the valve cover to the air cleaner housing. Also there are a lot of motorcycles that use this concept, i.e. the Suzuki DR line. It sounds like a great idea that someone else should have come up with. I'll be going to the hardware store tomorrow!

Ever look a car race motor, they dont vent into the intake, the EPA requires autos and streetbikes to do it for emissions. The air in crankcase is full of blowby and oil contaminated air, not fresh outside air.

[This message has been edited by F-Pilot (edited 01-22-2001).]

Take it from a mechanic, Autos have to recycle the polluted crankcase air by law. the 426 has what used to be called a road tube because it just dumped it out on the road. There should be pressure at the hose unless the engine stalls, and possibly creating a vacuum because of the temp. You will never see a race bike or race car with a pcv system unless required.

I would tend to agree that a PCV system will decrease the volumetric efficiency of an engine. Bad air.

My two cents.

Thanks for the support. I'll wait for an apology from akg800 :)

I do apologize F-Pilot. It was kinda fun using you as a punching bag for a minute or so. Although I do apologize I still do not agree completely with any of the posts here. Before I went and performed this operation, I went to a true expert, eric gorr, and asked him the question. His reply was that he would be concerned about the vaccum pulling oil into the airboot and also that the hot fumes would pre-heat the intake charge. Based on his opinion and expertise, I scrapped the whole idea.

The only problem I have with these BB's is that you can easily get sucked into some other "experts" thinking and not be objecive or independent. Alot of the info. is great, but you can't just jump in and read through ten different opinions about a subject and forget why you were there in the first place. You can drive yourself nuts! Anyhow, Peace! Later :)

with you all the way 800.

people seem to think were running mclarens here! first of all your not required to feed the air at the filter, or around behind the filter. were trying to keep the **** out the engine at BOTH ends. so just vent it into the bottom of your airbox & your done.

none of the above points were wrong, they're just not valid.

Keep It Simple Stupid. KISS

anyone who comes back at me, had better have done taf cam timing, run a dyno, have a 440, different cam grind, racing carb, vortex ignition, had the engine blue printed, head flowed, ported, big valves & even bigger balls!!!

you done the above THEN you can talk about bad air.


Taffy and 800,

Sorry that you guys don't like the advice you receive here on this BB.

Perhaps this will help.

The air that vents from a crankcase is referred to as "blow-by", and it's more of a "gas" than it is an "air".

The term blow-by is appropriate since additional gases are generated in the crankcase from the lack of a perfect seal between piston and cylinder. This gas is rich with hydrocarbons and dioxides making it unsafe for the environment.

Beginning in the 70's, the EPA mandated that certain automobiles re-circulate this gas back into the engine. Now a days, all vehicles, including street motorcycles are not allowed to freely vent these gases.

Racing applications, and some dirt bikes, are not required to re-circulate this gas. In many cases, the gas is released into the environment. However, many motorcycle and automotive race organizations require that a capture bottle be included so to avoid any oil from escaping and contaminating the track.

In some performance applications, (with more than one cylinder) a vacuum is applied against the crankcase vent. It is believed that this increases horsepower on the assumption that a piston traveling on the down-stroke will not be met with opposing pressure.

Since this gas is warm and contains contaminates that are not healthy to the atmosphere of an air box, there are no benefits to venting this gas back through the engine other than water proving and emission concerns. Since engines produce more power with lower intake manifold temperatures (hence the use of inter-coolers), in combination with the containing contaminants, a decrease in power would be imminent with such a configuration. In short, this is “bad air” and would have dramatically lower oxygen content than fresh air entering the top of the air box.

Obviously, the YZ 400 and 426 is designed for racing applications where deep water is not expected. For those that are using this bike for other means, they may have to consider routing this hose to a level higher than expected water levels. In these cases, the best configuration is a Y adapter at the top of the head with one hose running to the bottom of the bike with the second hose running up near the top of the tank or forks. This allows any oil that exits the crankcase to be relived out the bottom of the machine while assuring that a vacuum cannot be present when the lower hose is dipped in water. Without making any calculations, I would guess that the hose that runs upward would not have to be any larger than ¼ inch.

The bottom line is that many of the guys on this BB have been working on bikes, cars, and engines in general for nearly all their lives. Remember that good engineering is only a foundation, but application is what wins and that’s what these guys have. Be careful who you consider an expert.

Hope this helps and best of luck.


Hey DaveJ

Maybe yuo have read too deeply into my comments or I didn't elaborate enough. I appreciate a majority of the info. on this BB and have used much of it to set up the YZ I just bought. I myself am a mechanic and am very "experimental", with a methodical approach, when it comes to bettering my machines. I also appreciate when someone tells me of their experience, that they can back it up, almost 100%, with the complete what, where and why's. For example, your last post was one that I read start to finish and learned something from. After talking to Eric Gorr, I was planning on running a "T" system just like you mentioned. Anyway, I'm usually in here every day and hopefully will keep on learning from fellow "doers" in the future. Also I'm glad to see that Taffy was on my side in this little tiff, I definitely respect his posts. Keep an open mind! Peace!

Sports Administrator? Keep it simple stupid? A 440, diff. cams, big valves?

gosh did i start all this?

always a trouble maker i am!



& that's being kind to you!

you've totally lost the plot.

your head is deeply recessed up your own orifice.

if he rides into a puddle & his 1/2" ID blocked by the flow's of the nile your piffling 1/4" pipe isn't going to do him any good is it?

when did you experiment with that one!!!

have you ever tried running with a gas mask on or your mouth closed?

in BHP term's sandracer isn't going to notice a thing. your reasons for not routing the tube to the aitbox carry no weight in the day-to-day running of his bike.

my point being that unless you have bothered getting every last BHP from your bike why would you worry?

i welcome you to continue your line of thought so that we can all confirm that you're completely off your trolley!


"fight fight fight"

Taffy - you have again reaffirmed your place and rank in the world. All of us appreciate that at the present, and for all future references.

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