2006 YZ450F Oil Coming Out Of Breather Tube

1 reply to this topic
  • gmoney911

Posted January 16, 2009 - 11:15 AM


I just went on a 3 hour ride and when I got back home I noticed there was some oil coming out of the breather tube. I was wondering if this is normal and what causes it? Or do I have a problem on my hands? Thank you.

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  • grayracer513

Posted January 16, 2009 - 01:54 PM


It is normal. The piston displaces 450cc of air under it as it goes down, just like the air it displaces over it when it goes up. You can look at it as if it had an intake and exhaust stroke underneath the piston as well as above it. It is the function of the breather tube to allow that air to move in and out, at least part of it, so as to reduce the pressure in the crankcase. The other reason it's there is that the rings leak, no matter how good they are. They don't leak much but they do, and the combustion gases that get into the crankcase have to be allowed out.

So, at high speed, there's a lot of oil flying around in the cam box, and some of it gets caught in the air flowing out the tube during a pressure pulse, and if it gets far enough downstream before the pressure reverses to a vacuum, it will stay in the tube. Eventually, this builds up to the point that there's enough to drip on the floor, and there you have it.

Other things that are normal include your header glowing red (visible in twilight), or the radiator overflowing, either one after more than 90 seconds of idling.

BTW, welcome. Read the rules, read the Common Threads Sticky, learn to search, and enjoy using the forum. :thumbsup:

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