oil in breather pipe yfz450


Just bought 05 yfz 450,rage built national motor,480 athena kit ,wr crank.Can anyone tell me why we would have oil coming up breather tube that goes into air box?Also has a kn air filter with 2 sponge filters overtop.Too many?Any way to distinguish if indeed it has an athena kit by looking at the outside?

thanx for any help in advance


to much oil in the crank or excessive high rpm's....or a combination of both

thanx for replying.This started suddenly after riding in wet ,muddy conditions.Researched the subject and sounds quite common,but seems excessive in our case.Rode for 1/hr last nite and swingarm has fair bit of oil on it.Enough to drip off onto the ground.Anyone experience this?Appreciate any input on the subject


exact came thing is happening to me, it starteed after the first time I cleaned the K&N filter. I mean there is a lot of oil on the swingarm.

Sounds like this is a common thing .Doesn't seem normal to me.Does anyone know if there is a fix.Is the motor tired?We had a fair bit of oil on the arm also.Not sure on the air filter issue,seems like overkill.I was wrong when i said it was a k&n.After pulling off the first foam filter the inside says pro flo or something like that.It has its own foam built in.Do I need the second one?

Thanx for any help


thanx for replying.This started suddenly after riding in wet ,muddy conditions.Researched the subject and sounds quite common,but seems excessive in our case.

The YFZ450 holds 1.98 quarts of oil... and accepts less than that when doing an oil change. There's a good chance you overfilled it. I always follow the manufacturers directions on adding oil to the crankcase and the resevoir and have never had a problem.

That was the first thing I checked because I have had this with a ktm product that suggested 3/4 liter when changing oil but would spew out of the breather if you used that much .Less oil no problem.I did check ours and was right on the full line.Haven't changed it yet.Was wondering if I had another ktm on my hands.It's also been suggested to me that the motor is pooched and this is blowby.Seems like a lotta blowby to come on suddenly, and I don't have anything coming out the pipe or driveability issues.Pulls like a bear.That's why I'm using this site .Let the more experienced guide me.

Thanx for the input.Wanna go racin this weekend and don't know if we have a problem or not.


I assumed this was a fresh motor since you just bought it. Sounds like you got a beat motor. If the oil in the crankcase smells like fuel, that's your second clue there might be blow-by.

No, the oil is clean and does not smell like fuel.This happened suddenly,only noticed it after the really wet ride,but not before.Don't think we pulled in any water but it was my son riding it not me.airbox is open and has 2 large openings at the bottom also(?)Thinking it might be time to open it up.Motor was not fresh but was said to have maybe 20 hrs.Never know when buying used.Everyone but a few is of the opinion this is normal?I'm not so sure


Everyone but a few is of the opinion this is normal?I'm not so sure


Maybe it's common if you overfill it? I'm riding with a few YFZ450ies this weekend. I highly doubt any of them will demonstrate that problem.

Well I'm guessing by your last post that we have a problem then.Changed the oil and filled to 1.98 litres and we ended up being overfull after warming up.Installed longer tube from seperator and didn't have any leakage.Tough to decide if we should have to tear into the motor.That kind of crankcase presuure would indicate perhaps a cracked piston or poss h/gasket leak but we don't show signs of your normal worn out eng symptoms.

Thanx for your help


I found this on the Duncan Racing web site - for modded yfz motors.

OIL BREATHER: It is also advisable to re-route the exit breather line going from the oil breather catch tank

(mounted LH side of sub-frame), it is no longer advisable to have it vent into air box.

It is NOT recommended to remove this breather catch tank.

It is recommended to install a longer hose on the OUT line of the catch tank and route it over top of engine

but below fuel tank to front of engine and have it stop in front of radiator. It is also advisable to install a

universal type paper filter on end of this vent hose. These types of filters are commonly used on 4-stroke

engines of all types.

Make sure that ALL of the engines breather hose lines are free of kinks or any other types of obstructions.

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