1998 YZ400F Cylinder Breather Compression

1 reply to this topic
  • Meaty400

Posted June 13, 2005 - 08:08 AM



I recently upgraded from a CR250R to a 1998 YZ400F and I am loving it.

I have been reading through the forums, and have found some of the answers to my questions, but just wanted to verify. I too have a little oil drip from my cylinder breather hose. I purchased this bike used from a dealer, and the bike is in almost mint condition and I am almost positive the bike was never raced. Not fully knowing how the bike was service I did a few quick tuneups like changing the oil to see if that helps.

What I am wondering is there suppose to be some compression coming out of the breather hose?

I never really noticed it until the otherday when I was out in the field and saw the grass moving under my bike. It's not a lot, but you can feel it if you stick you hand under the hose. Just thought I would ask the experts here, cause I love this bike and don't want to riding it, if it needs some work.

Meaty 400 :)

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  • grayracer513

Posted June 13, 2005 - 10:57 AM


What you need to realize is that the piston displaces 400cc on both sides of the crown, top and bottom. In a two-stroke this is a part of the engine's operating principals, generating the vacuum and compression that give it the intake and transfer elements of the cycle. In a four-stroke, the air under the piston is just in the way. The breather is there to give it somewhere to go.

At an idle, this is usually a pretty much zero sum deal, with air being pumped out of and drawn back into the hose. Add any kind of load on the engine, and you have some combustion gases slipping past the rings and adding to the air mass in the crankcase. This escapes through the breather as well. Now add oil vapor whipped into the crankcase air by high speed operation, some leftover part-burned fuel, to what is now a net outflow of air, and you soon have a coating of goo on the interior of the breather hose, some of which drips off onto the ground.

All normal. :)

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