WR400 - air pulsing from the valve cover vent hose

6 replies to this topic
  • hozhead

Posted April 14, 2009 - 01:45 PM


I have a 2000 WR400 that is running the auto decompression exhaust cam from a 450.

I did the mod two years ago and have never had problems with how the bike runs but, a two weeks ago I put in new piston rings (Wiseco piston, stock size & compression) because the oil ring hadn't seated very well previously and I was burning quite a bit of oil when riding.

Swapping the rings was easy and really only took a few hours for the full teardown and re-assembly of the cylinder and head but now, I'm getting blue smoke from the exhaust when I fire it up (it gets better but doesn't go away 100% after warm up and riding).
I also noticed today that there is air PULSING from the valve cover vent hose - the pulsing is consistent with the bike's RPM. If there is loose dirt on the ground below the bike, the vent hose will actually blow it around.

I thought perhaps I had the timing a tooth off and was getting some blow by but, I just opened up the valve cover and found that my timing is accurate based on this: http://www.thumperta...078#post3706078

Does anyone have any suggestions of why there is so much air being pushed out of the vent hose? I'm relatively sure it will be the same issue causing enough oil to get burnt and cause the blue smoke.

  • hozhead

Posted April 14, 2009 - 01:49 PM


I forgot to mention - the bike still runs fantastic and starts pretty easy too.
I've only ridden the bike around the neighborhood for a while since swapping the rings and haven't put it through any dirt use.

  • William1

Posted April 14, 2009 - 02:35 PM


You are noticing a normal event. You have a big old piston going up and down. At slow speeds, there is time for the air to go in and out. At speed, it tends to stay in the engine somewhat. You may want to either put a filter on the hose or vent it into your air boot.

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  • matt4x4

Posted April 15, 2009 - 06:01 AM


When venting it into your airboot, you will need to make sure it runs downhill all the way to the boot - any uphill runs will collect oil at the lowest point before the uphill run which can eventually clog the breather.

  • William1

Posted April 15, 2009 - 07:19 AM


Regarding the smoke on startup, sounds like time for a replating of the bore and fresh rings. Send your barrel to Millenium Tech.

  • ss396

Posted July 22, 2010 - 07:18 PM


my 2000 wr 400 did the same thing lapped valves and new valve stem seals should do it

  • jonooffler

Posted July 24, 2010 - 11:14 PM


Hi my WR426 blow a lot of air out the breather pipe and also drips oil out of the breather when she cools down after a ride, I have been told that it normal.


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