Crankcase Breather Pipe

:):D OHHH YES please get the part no# I have tried to do this but get a proper Kink in the pipe where it comes out of the rocker cover so ended up shortening the pipe to prevent "Suck"

If you could get the part no# that would be Cool :D:D

[ January 19, 2002: Message edited by: Guy ]

I've heard stories of guy's starting their bikes in water (having first stalled them) and sucking up water through their crankcase breather pipe and hydro locking the engine. I've always repositioned my pipe from the front of the bike to the rear just at the back of the carb. Today a pal of mine had a brand new WR426 and it came with a spare second pipe so you have the option of re-routing the pipe into the top of the air box seems like a good plan if any one is interested i'll try and get the part number

You can put a PCV valve in also. lots of guy,s have done this. If you go to the archives, a part number was given. By the way, the water thing, Been there, Done that! It ain't pretty.

I will look up the part number this week I fitted this pipe last night, it comes out of the crankcase and across the frame then over and into the top of the air box you need to drill out the plastic bung and it slots straight in. When you fit it you can see clearly that the bike was designed for this pipe i.e the tank is shaped for the pipe to fit as standard and the top of the air box allready has the opening for it, can't understand why they run the pipe down the frame and out at the bottom in the first place!

Ron, do you know what pcv valve you ran?

Hey, just to let you know...... on all Buells the crankcase breathers (two due to two heads) are similar to my WR426, and they are routed to the airbox. This causes all kinds of rideability problems. Oil will get misted through this tube and into your intake system, when you hit the throttle hard a blast of hot air from the engine is sucked directly into your intake causing wierd jetting issues. Beleive me, this is not worth it. We worked long and hard to come up with a solution to this problem and the only one is to route your breather to atmosphere, however, the EPA will not allow it on street bikes so customers must do the mod themselves. With all that said, I would simply fit a MOTORCYCLE approved PCV valve (i.e. one that will work in any position) to the breather and still route it to atmosphere. This valve will allow your crankcase to breath properly but will only allow air to escape and not to be sucked up, therefore, no water can get sucked up either. Good crankcase breathers are available at and for $80 it is alot easier than routing it to your airbox and having trouble. The other added feature of these PCV valves is that they will never stick closed causing catastrophic engine damage. Beleive me on this one, or if you dont post a message about crankcase breathers on or



P.S. if you have any questions about this issue and want to talk more indepth just drop me a note.

Thanks, you have just saved me the time and trouble in fitting the alternative breather pipe. Following on for what you have mentioned that is why I when opening the Street legal kit I found a odd shape rubber pipe.

:) Ronin, I do hear where you are coming from, but my thoughts are that...every road bike I have ever run has had it's breather fed back to the airbox, no detrimental effect noticed, There is an amount of oil mist deposit but nothing to write home about, I have never run a Buell, (not my cup of tea old bean!) so I can't comment on them, I will try this mod because it seems good to ME, after all if we were all the same the world would be really boring! :D

If you are correct and it "runeth like the proverbial swine" then I will be the first to hold my hand up and say..."Well Ronin was right"

so we'll wait and see! :D:D

OK, so we're worried about sucking water up into the crankcase vent hose during deep water starts. I'm not sure a PCV valve is the best solution to this problem.

I replaced the stock breather tube on my '99 with a clear vinyl tube (it's all I could find that was the exact same dimensions) and routed it up near my steering head to keep it up out of the water. As you might expect, it made a mess as it spewed oil all over my radiator guards and frame. I wanted to get one of those little UNI air filters for the end of the tube to filter the dust and capture some of the oil but I was too impatient wait while it was being ordered. So I stuffed some polyester batting into the end of the hose as a substitute and was considering a PCV valve. What I noticed was that while the bike is running, the air flow through the breather hose travels both directions. Since the hose was clear, I could see that little wad of batting vibrating back and forth about a half inch in each direction. Now correct me if I am wrong, but doesn't a PCV valve restrict air flow to only one direction? I wouldn't want to choke off the air flow and cause some kind of piston lock or something.

Someone suggested that a better alternative may be to put a little hole in the breather tube up near the top of the bend in the hose. The oil would still flow down the tube, and the hole would break the suction in water. Cheap and easy.

I havn't tried this yet because I figure it's better to just push the bike out of the water and THEN start it!

:D Rich, the only problem with routing it up round the steering head is that any moisture vapour trapped in there when it condenses, can only run back down into the head :)

I believe that someone else, has observed this happening and posted it on the forum, As I stated above,The Big "H" (Honda) use both methods in conjunction, a line straight to the airbox so the motor can breath clean (or blow to it), with a Tee off with a valve so that oil/water etc can be vented without the danger of "Suck back"(and we all know how nasty "Suck Back "can be) ROFL!


[ January 21, 2002: Message edited by: Guy ]

Pro stock drag bikes use vacuum pumps to evacuate the crancase pressure and add hp.

The pumping beneath the piston is not really controlled just vented on our bikes.

It has been a proven method of adding hp on sportsters like Ronnin pointed out.

I have always been curious about running a pcv valve on a wr and would like to hear peoples experiences...

:D OK I can see that as well, but if you take that thought and move on, the vacuumn caused in the inlet tract, will surely have the same effect on crankcase pressure as a vac pump(albeit to a lesser degree) as it "sucks " the vented air out of the crank breather?? The question only remains as to whether, the warmed air will have a detrimental effect on the jetting,

The only other point is that, drag bikes, buells etc, all run much bigger pistons, crankcase pressures to a 400/426, rev different etc, so what applies to one, might not apply to the other!

My Kawasaki ZX7R had Ram Air, now that is an increase in airflow,pressure etc,and that ran OK :)

My .02 worth.

You all bring up good points. My unfortunate experience shows you don't have to try and start the bike to suck water. Just stalling in a medium-deep creek will do the job! :)

On KTM 400 and 520 E/XC's the breather is routed into the airbox and DOES cause drivability and jetting issues. I don't think this is a good solution.

I ran clear hose from the breather and ended it behind the carb and above the chain to keep the mess in one area.

Happy trails,

I've stalled my bike in some pretty deep water and as long as the airbox stayed out of the water, I didn't get any water in the motor. I ended up returning to the original breather hose configuration because I didn't like the mess it made. I have also rerouted two of my carb vent hoses into my airbox and I put a tee in my carb bowl vent hose and ran one end into the airbox as well. Again, as long as the top of the airbox stays above water the bike runs just fine. I've had stalling problems in deep water before I did that.

That said, I've also dropped my bike in 3 feet deep water and filled the motor with water and mud. :) I had to have someone pull me to start it after I spent an hour with the spark plug out kicking it to push out the goo. It was like a geyser!

:D OH dear, the only time I've seen a bike that bad was a mate's Suz DR350 E-start, riding thru a flooded lane, Hadn't realized he was on the centre of some wheel ruts when he dropped off in to about 5' of water deep enough for him to swim anyway, all we could see of the bike, as it was leaning up against the side of the track was a handle bar andthe instruments lights.

I took us over two hours to start it, we ended up standing it up-side down to drain the water out, Not Good! :)

About the airflow of your crankcase breather....Yes, it blows both ways without a restriction device. To the best of my knowlege the reverse flow is not needed to deter engine damage. I beleive that when the piston travels down it causes high pressure in the crank and when it travels up it causes a low pressure event. On a dry sump system if a continous high pressure event occurs (clogged breather/stuck PCV)it can cause the oil system to lose it some of its ability to pump oil to the engine and also cause the piston to stick at top dead center. This can cause extreme damage to the engine or at least in H-D powerplants. When someone puts a PCV valve inline of the breathers you do not get near the air out due to the fact that you do not suck air back in every time the cylinder travels upwards. I also have placed clear tubing on my Buell to see what happens and it does move both ways. Of course with a PCV valve in place it only travels one way. on another note, if you guys raise your crankcase breather up to a point higher than the crankcase or put a dip in it I think that eventually the water moisture may build up and cause the engine to suck up the water/oil left in the hose. just something to think about.

What I have done this afternoon while installing my graphics is to install the same style PCV valve inline with the crankcase breather leaving the routing the same for the breather line including the final location of the end of the hose. It seemed to work fine and i must say that when I put the tube in a 5 gallon bucket of water, held the kill switch down and kicked it like a sonofabitch for 10 minutes no water made it past the PCV valve, however the water did hit the valve body. (I used a friend and clear tubing for this test)

Just my thoughts and tests, take from it what you will.




Excelent postings, I too blocked off all vent to airbox then to atmosphere. when racing ZX10's Simple way to put it is..

Running vent to airbox makes your bike eat it's own poop! Just like epa likes it..Yech! :)


Thanks SoCal, I love the analogy. It just pains me to see the posts about putting the breather into the airbox, us Buellers take so much time, effort, and cash to properly remove that crap from our bikes it almost strikes me as blasphemy to think about putting a crankcase breather into an airbox of a bike that already has its breather vented to atmosphere.




Hey Ronin,

I too have seen an obnoxious amount of crud in my Harley and Buell's airboxes. Hell, I don't even oil my K&N on the Harley because the crank case does such a good job.

Seeing all the crud that comes out of my WR's vent tube makes me think it's probably as bad. I really don't like the idea of sucking all that stuff back into the box. It makes sense that the EPA doesn't want it going on the ground either.

I'm wondering if the tube might even get clogged up with the crud if there is a dip in the tube???

John, you may be right on the clogging of the tube, but maybe all the moisture keeps the crud from building up too bad. I too have friends who dont oil their K&N filters due to the oil spewage. However, I have them all on Hayden crank vents with a little K&N filter on the end of the breather hose. This setup keeps everyone happy. H-D makes a bridge for both the cylinders and then the bridge routes towards the back of the bike where you can put a vent and filter. We sell a ton of these at the dealership where I work. you might want to look into this for your H-Ds and Buells.



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