WR400 vent hose oil leak

My '98 WR400 is dripping oil from its vent hose (vent hose is connected to the top of the valve cover). It's not a continuous leak under the bike, but drop by drop it adds up and has me now concerned. It'll probably leak 1/2 quart in 2 hours of hard riding. I'm guessing that I'm not the first one who has experienced this and would appreciate your feedback to understand what could be causing this. My analyses would suggest that either:

- Piston rings are shot and some blow by gases leak into the crankcase which then could blow oil out of the vent hose.

- Or, some valve seal is shot and gas is escaping to the top of the cylinder head pushing oil out as it makes its way out through the vent hose (just read this possibility in this month's Motocross Action magazine).

Either way, I'll need to open the engine but I'm wondering what would typically be causing a leak as described above. Your thoughts?


Mine does that too an everyone says its normal. It realy subsided when I switched over to Amsoil 10w40 motorcycle oil. But from what I can tell they all do it even the Hondas.

The 400 and 426 all seem to do this but my 450 didn't. and yes it is normal

Thanks for the feedback -- I have a leak test scheduled for Saturday to find out how tight the valves and piston/rings are. Will see what comes out of this. My concern about losing oil the way my WR400 durrently does (1/2 quart in 2 hours of hard riding) is that there isn't all that much oil in the system in the first place... Then again, this is my first 4-stroke and I'm discovering things as I go.


WOW 1/2 a quart is a lot! Mine only loses a few drops. Sounds like you may have some blow-by at the valves. :D

Please follow up with thread with your findings. My 01 used to leak...until I re-routed the vent tube to the air box. :D

I'll follow up with the info found during the leak test. My current guess is that the rings are worm and the engine is experiencing some blow-by which exists through the vent hose with such force that oil is pushed out as well. Weird thing is that I would expect some major power loss from the blow-by but I don't find it noticeable. There is still all the power I can handle :D

had a similar problem with oil from the valve cover breather on my 04 450 subsequently freaked out and consulted my mechanic who informed me that if you drop the bike on the cam chain tunnel side oil can flow up the cam chain tunnel and trap excess oil in the head, the length of time the bike spends on its side also affects how much oil transfer occurs. the result is the oil that hasnt been drawn back into the oil system gets spat out the breather.

maybe the oil is over filled... also, I wouldn't take it apart if it's running fine.... if your valves are off, it will run like crap

my 2 cents

Just did a full oil and filter change and now used Honda 10W40. Went for a test run around the neighborhood (neighboors love it when I do that, really :D ). Interestingly the vent hose oil leak seem to have subsided, at least it's not as obvious (only saw one drop). I had full sunthetic before -- weird, could oil type do that?

I'll still have the leak down test done at the local Yamaha dealer tomorrow to get a more complete picture.

Just did a full oil and filter change and now used Honda 10W40

OMG.... you put Honda oil in a Yamaha??? :D Shut it down quick, IT GONNA BLOW!!!! :D

he, he :D Was wondering why the oil dip stick resisted so much getting back into the oil tank. And I thought it was just me with a six pack related hallucination :D

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