Someone please explain function of the valve cover breather hose

26 replies to this topic
  • Drtbykr

Posted August 02, 2004 - 05:40 PM


I would like a detailed technical explaination when someone has time. I was at the Canadian Nationals yesterday and a bunch of fast Intermediates asked me if I wanted to go trail riding with them again (big fun keeping them in the same time zone, but I can always pull the age card). Would have loved to but the bike is in the shop until Wednesday. (have Visa, will ride)

Anyway, they were asking what was wrong with my bike and I told them that I had packed the motor with water and grit since I was in a mud hole, stuck (about 7-8 inches deep) and repeatedly tried to start the bike. Big mistake,motor needs disassembled and cleaned a new rings).

These guys (and one LOVELY lady) all have CRF's. They said that the Honda had a plug in it. Is this like a check valve or PCV valve or what. Does the Yamaha have anything in its hose?

I notice on the Yamaha web-site this hose goes to a breather assembly on the YZF450 quad. And if you route it directly to the airbox without a tee, would unwanted oil be placed into the airbox, or is the amount so small to be a non-issue.

Does anybody know why there are so many different ways to do the same function, and in your opinion, did Yamaha drop the ball on placing a free flowing hose to such a low and vulnerable location to do this function?

Thanks for your time.

  • Indy_WR450

Posted August 02, 2004 - 06:00 PM


You need to vent the head to prevent blowing the cam covers off during normal engine operations. You should route it to the air box and dont worry about oil getting in the air box. You would have to have the bike upside down for a while to get the oil to flow into the air box. :thumbsup:

  • yz426onNOS

Posted August 02, 2004 - 08:39 PM


The husabergs have had problems with oil going into the carb because of routing it into the airbox. Just route it straight up and put a filter on it. Here is a pic of it done on a berg. Thanks to

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  • ovrrdrive

Posted August 02, 2004 - 10:30 PM


I was toying with the idea of rerouting mine last weekend, but opted to cut a hole in the hose about halfway down the head instead. I've seen where a bunch of people did that and it looked like an easier idea to me.

The hose isn't going to suck water with the hole in it, and this way I don't have to worry about oil in the airbox or backing up in the hose due to it being horizontal.

Like I'd ever ride it in water anyhow... :thumbsup:

  • tcmII

Posted August 03, 2004 - 04:09 PM


Reading this post I got an idea. The hose serves the same purpose as the pcv system on a car, which is to allow the crankcase to vent rather than build up pressure.

Cars have a pcv valve that allows the crankcase to vent (hence the name - positive crankcase ventilation), but not draw air. In other words a check valve.

I can think few different valve styles that would fit the stock hose. There are some valves that would splice into the hose or plug into the end.

You could also add a breather filter to the end of the hose if you ran it to the air box and it would trap any oil that came from the hose. Again, just like a car.

I bet you could find everything you need at Murray's for less than $10.00.

  • dkwok

Posted August 05, 2004 - 10:07 AM


where exactly did u cut the hole?

  • Grandville

Posted August 05, 2004 - 11:57 AM


This is an interesting topic. Ever since I routed the breather tube into my air box, I have made several observations; 1)unlike a car's breather device, our breather tube is very active pushing and pulling air into the top end. The greater activity of air pressure is because our engines are single cylinder and have no other cylinder to equalize the air pressure on an opposing stroke, 2)Yes, this tube is discharging a small amount of oil in the air box. This could cause problems in gumming a caburetor and evntualy affect performance. But, periodic box and filter cleaning can avoid these problems.

However, modifications should still be considered because dirt or other harmful stuff is being sucked into your engine through this breather tube. Like YZ426onNOS suggested, this breather tube should be filtered. I put a rubber band around a car's breather element over the tube and I was done. Here's my question. What impurities are in the air that comes out of that breather tube? And then goes back into the carburator going to do to our bike's engine performance? I don't have these answers, but my bike seems to be digesting them just fine.

Oh-by-the-way, If your going to put a hole in your air box to re-route your breather tube, use a hot bolt to melt a hole. It is a lot easier than removing the box to drill a hole or stabbing it with a knife.

  • 5valve

Posted August 20, 2004 - 12:03 AM


euro WR have factory fitted breather hose to the air box

on my last oil change I dropped a bit too much oil into the frame
during the course of 500 miles it slowly spat all the excess oil through the breather hose, soaked the airfilter, came through it and periodically I freed the plug on the lower side of the airbox, so it could pour out
when the oil level reached the MAX line it stopped filling the a. box

  • WR_Dave

Posted August 20, 2004 - 04:22 AM


Just put a plastic"TEE" fitting in the breather hose at about the valve cover to cylinder head area and route another hose from the tee up to the steering head area. I also did this with the two carb vent hoses that cross over the front of the carb and all your water sucking and stalling problems are over! WR Dave. :thumbsup:

  • Sylvain

Posted August 20, 2004 - 06:12 AM


About :

euro WR have factory fitted breather hose to the air box

Could you show us a picture of the factory set up ?

I wonder why Yamaha didn't standardise this set up ?
I beleive that water crossing in America is about the same as water crossing in Europe. :devil:

So why the difference ? :thumbsup:

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  • 5valve

Posted August 21, 2004 - 02:50 AM


I've rerouted mine out of the air box, but I can take a picture of a plug and post it

the diffrence is because our water has thicker molecules :thumbsup:

well, the true diffrence is because of environment censerving law, i think EURO 5, wich doesn't allow the excess oil to be leaked into the ground (1 litre of oil contaminates 1000 litres of water)

  • Sylvain

Posted August 21, 2004 - 04:26 AM


You're absolutly right ! I remember reading about environnement issues in Europe and to say the least, you guys are way ahead on this. :thumbsup:

  • 5valve

Posted August 21, 2004 - 05:18 AM


the "way ahead thing" comes in quite costly for the customers, the environment taxes are much higher here :thumbsup:

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  • simch

Posted August 23, 2004 - 08:30 AM


Cutting the hole half way down seems to be a good idea. I spose you could just cut it shorter so there is more clearance to the ground too. A bit of oil driping on the motor is a small Price to pay.

WHY OH WHY did Yamaha run it down so low?? Unless it needs to vent under the "sump" for some reason??

  • ncmountainman

Posted August 23, 2004 - 08:49 AM


i ran mine over the carb and just cut it off so that it stops behind the frame and if it drips it goes right on the chain(have yet to see it drip)and if you stall in water that deep you can give it a tug and point it straight up like a snorkel :devil: don't forget to reroute the carb vents! :thumbsup:

  • vtfootball79

Posted August 25, 2004 - 02:36 AM


One day I was out riding and stalled in a mud hole, while trying to start the bike I heard suction and remember about the posts I had read about on TT. So I luckily stopped before the hose could suck up anything all the way and pushed it out and then started her up on dry land. When I got home I hacked off a good portion of the hose and have no problems since then.

  • bdmmotard

Posted January 08, 2005 - 10:08 PM


Thanks for the info guys.

I have a bit different of a problem. my bike, 98 wr400 does spit a decent amount of oil. problem is i use it for supermoto, and oil on the track/outside of the sump is frowned upon for obvious reasons. currently i use a baby bottle catch can, but it's still messy. should i run it to the airbox? i figure the tube angling up more would allow breathing but have less oil spitting out? thanks for your help!

  • simch

Posted January 09, 2005 - 02:44 AM


Which side of the bike is the head breather visible from, or is it off the top. I have a euro spec one, and cannot even see the hose at present!! Do I need to remove the tank, or is the routing different on mine??

  • Hamish

Posted January 09, 2005 - 03:08 AM


the exit of the breather hose is just below the LH footpeg.
It comes from the top of the valve cover down the front of the engine.
A euro spec bike MAY have it routed to the airbox.

  • toyota_mdt_tech

Posted January 09, 2005 - 04:30 PM


Thanks for the info guys.

I have a bit different of a problem. my bike, 98 wr400 does spit a decent amount of oil. problem is i use it for supermoto, and oil on the track/outside of the sump is frowned upon for obvious reasons. currently i use a baby bottle catch can, but it's still messy. should i run it to the airbox? i figure the tube angling up more would allow breathing but have less oil spitting out? thanks for your help!

Use an oil trap. Just fix a container with 2 pipes coming out of the top of it, hook the hose fromt he valve cover to one end and then vent the other end elsewhere, even higher up. Any oil leaving would cat caught in the trap, and any water trying to get in would also be caught in the trap. Just service the trap when you service the bike.:cry:


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