oil coming out of vent tube

7 replies to this topic
  • section 8

Posted August 13, 2007 - 03:52 PM


ok im new to the 4 stroke's and i am not very good at working on them. i tried searching for my question and nothing popped up. i recently bought a 2004 450f and i rode it for about a week. the last time i rode it it started leaking oil out of the head vent tube. does anyone know what this could be. im thinking there is too much oil in the bike but i dont know. :excuseme:

  • grayracer513

Posted August 13, 2007 - 04:36 PM


Some oil out the breather is entirely normal. By that I mean 1-5 or 6 drops out the tube every time you stop the bike after a hard session. Some as you ride, too. If it's excessive, it can mean there's too much oil, but you have a dipstick, so use that. Because the bike is a dry sump system, you can actually overfill it by as much as 400cc and not cause this problem (but you should be careful about opening the dip stick up).

The other thing that can cause this is not so innocuous; weak rings. If the rings don't seal well, there can be an excess of combustion "blow by" in the crankcase, and that will carry more oil out the breather than normal. A leak down test will verify whether this condition exists.

  • section 8

Posted August 13, 2007 - 06:48 PM


thanks man , this might sound stupid but what is a leakdown test

  • grayracer513

Posted August 13, 2007 - 07:57 PM


A leak down tester consists of an air pressure regulator going to a gauge, then an orifice, then a tube leading to the spark plug hole with another gauge on it. The engine must be locked at or near TDC ( I personally think that 10 degrees off TDC will give the most meaningful results, but it doesn't matter that much).

A set amount of air is fed against the orifice (usually, 100 pounds, because it makes the arithmetic simple) and the percentage lost to leakage is noted. For instance, if you feed 100 psi to the input side, and the second gauge shows 97-98 psi, your engine is in pretty good shape. 95 or so, it's a little tired. This test also makes it obvious as to where the leakage is.

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  • trevorchappell

Posted August 13, 2007 - 09:31 PM


did you make sure that you did not over fill the bike with oil??? sounds like a stupid question to ask but i have seen so many people do this and it will make it put oil out of the breather hose

  • yellowdub69

Posted August 14, 2007 - 06:19 AM


I have the same happening on a 1998 YZ400, it puts out about 5cc for 20 mins of hard running (I have to run a catch tank for racing so I can measure it) and I have put that down to the rings being worn. I have no problems starting yet, just hoping it makes it to the end of the season for the winter rebuild.

  • tutta_velocita

Posted August 14, 2007 - 01:23 PM


Mine used to drip a little out of the vent tube as well, but hasn't done it very much since I stopped riding at such high rpms. I ride at lower rpms and it just seems like it doesn't leak as much.

  • Fastest1

Posted August 19, 2007 - 06:32 AM


First when checking the oil. Bring the bike to operating temp, kill it, wait a minute, pull dipstick and check. Dont do it after sitting for a while or you will overfill it slightly. If you are winding it up and riding at the upper rpms (not necessarily fast maybe just a low gear) it will drip more than if shifting earlier. The rest regarding rings and leakdown is spot on.

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