yz400f crankcase breather question

2 replies to this topic
  • mediumdaddy

Posted October 09, 2012 - 05:25 PM


So i posted before about my 99 yz400f blowing oil out of the crankcase breather hose (i think this is what hose that is) i started it up today in shorts - dont yell at me i was only riding it up the yard so i could wash it- and i could feel air blowing by my left foot. I started to feel around, and there was a lot of air pressure coming out of the breather. it seemed odd to me, so i plugged it with my finger, and the bike didnt seem to run any differently. Is this normal? knowing my previous luck with this bike, probably not.

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  • DeeCee

Posted October 10, 2012 - 04:02 AM


There will always be some pressure coming out of the breather as well as a few drips of oil.

By relieving the crankcase pressure, leaking joints/seals/gaskets are avoided.

Most later model bikes vent the breather into the induction system so owners do not realise what is happening and do not raise any concerns.

In the YZ400's case, there is no such routing of the breather into the induction system, instead there is a vent direct to atmosphere. There are numerous posts on ways to undertake such routing if you want. It is however not necessary.

It should not be a concern unless there is excessive oil discharge (usually caused by overfilling) or smokey gas discharge which can be a sign of piston blow by.

If the bike runs OK and doesnt burn a lot of oil, don't worry about it.

  • grayracer513

Posted October 10, 2012 - 05:57 AM


The piston in your bike displaces (moves) 400cc of air. The thing you're missing is that it does that on both sides, top and bottom. When the piston comes down, it shoves 400cc of air out of the way, and that has to go somewhere. When it goes back up, the vacuum wants to be filled. That's why the breather is there.

What is also present in the crankcase is any combustion gases that leak past the rings, which can be a pretty fair amount of vapor in an older bike with tired rings. The flow from the breather is mostly an equal amount out and then back in, with the combustion blow by creating a small net outflow.

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