YZ400F Oil Leak / Airbox

After riding my YZ400 the other day I found oil on the ground next to it after putting it away in the garage. I cleaned it up, but the next day there was more oil on the ground.

It appears to be leaking out of the air box drain tube, onto the chain and swingarm, then onto the ground.

Is this normal?

If it is how much should leak out?

I don't ride this bike often, usually during Green Sticker season when I can't ride my 250 2st. I did change the oil between the last time rode it and the most recent outing.

I am certain I filled it with the correct amount of oil, but let's say I didn't:

Would over-filling the bike cause it to push oil out that direction?

If so, is it harming the bike?


If the crankcase breather has been rerouted to the air box, this could account for the drip, and could even be fairly normal. The engine, especially one with some time on it, will push some oil out of the breather under normal circumstances, and a few drips is nothing to worry about.

Overfilling will cause excessive oil exhaust at the crankcase vent, though, but in a dry sump like the YZF, this won't happen until the oil tank is physically filled with more oil than it will hold. When this happens, the crankcase cannot be scavenged as it should, and the engine may smoke and blow oil. In extreme cases, it may blow back raw oil into the intake, and that can end up in the air box. Other than fouling plugs and generally making a mess, it does no real harm.

is it motor oil or air filter oil leaking?

If your crankcase breather is not routed to your airbox then chances are the oil is from your filter (especially if you recently re-oiled it).

If the crankcase breather is routed to the airbox then this is the most likely source of the oil.

Another possibility is (assuming the crankcase breather is routed to the airbox) the bike was on its side with the engine running (even for a short period) and some oil got pumped out the crankcase breather.

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