HEADS UP!!!

Site upgrade in progress... Core site functions are working, but some non-critical features/functions will be temporarily unavailable while we work to restore them over the next couple of weeks.

Please post any bugs you encounter, but before you do, check to see if it's already listed.

Thanks for your patience while we work to improve the community.

Sign in to follow this  
Followers 0
section 8

oil coming out of vent tube

8 posts in this topic

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:

Share this post


Link to post
Share on other sites

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.

Share this post


Link to post
Share on other sites

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.

Share this post


Link to post
Share on other sites

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

Share this post


Link to post
Share on other sites

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.

Share this post


Link to post
Share on other sites

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.

Share this post


Link to post
Share on other sites

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.

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!


Register a new account

Sign in

Already have an account? Sign in here.


Sign In Now
Sign in to follow this  
Followers 0