Crank vent tube still leaking oil after new rings

27 replies to this topic
  • MaxAZ

Posted October 04, 2004 - 05:47 PM


Just put new rings and piston in awhile back and was hoping to significantly cut back on leaking out the vent tube. I've always had what I thought was to much leakage so when I finally started to lose a little power I did the top end rebuild witha Weisco kit. The old rings were 2-3 thousands over max spec and piston and everything looked good. New rings measured within 1 thousand of minimum spec so I should be good and tight. I've now got about 9 hours on the motor so I should be broke in and seated good. Now I know these measurements I'm about to give aren't specific but please bear with me. After a hard ride, when I park the bike behind the truck, take all the riding gear off, [@#$%&*!] with the guys and drink a beer, I then go to load the bike and always see a oil stain on the asphalt about 5 inches in diameter under the crank vent tube. Then when I get home 90 minutes later, I've got a similar stain in my truck bed. If I didn't park a crappy piece of carpet under the bike I'm sure my garage floor would be stained too. Now I haven't yet measured how much oil I have to add to get back to level because I've still been draining the oil after every ride during the break in and it's worthless to try and measure what you drain out of the bike since some of it goes on the floor, frame, arms, shoes ........ Doesn't this seem a little excessive to some of you guys? I'd sure like someone else's opinion and while you're at it, do you think I've got enough hours on the motor now to go back to Mobile 1 synthetic? :cry:

  • skthom2320

Posted October 04, 2004 - 06:05 PM


My YZF never had more than a drop or two.

How much oil are you putting in your bike?

  • MaxAZ

Posted October 04, 2004 - 06:19 PM


Never more than 1.5 qt and always clean the Scott's oil filter. The manual shows 1.6 qt. but I've found that 1.5 puts it right in the middle of the dip stick and anymore than that really blows it out the crank vent tube.

  • sirthumpalot

Posted October 04, 2004 - 06:58 PM


This may sound like a stupid question but I have to ask; Are you draining both the frame and the motor? It's normal to loose some oil out the vent, especially if you spend a lot of time in the upper RPM range, but if you're getting a puddle then that does sound excessive. :cry:

  • Fastest1

Posted October 04, 2004 - 08:36 PM


You do start your bike and run it for a minute or two before you check the oil dont you? Had to try everything!

  • MaxAZ

Posted October 04, 2004 - 08:54 PM


Yes on both those points and there's no stupid points because I do know some people don't know to drain the frame and motor or idle the motor prior to checking the oil level. I just got into riding a few years back so I still remember the learning process regarding dry sumps. Other than worn rings, what are other causes for all this oil? I should add that the motor starts great and runs great. Never any problems. Love the bike but don't like the fact that it needs a diaper!

  • FFRacing79

Posted October 05, 2004 - 06:38 AM


Unlike the CRFs, the YZs have no baffle system in the valve cover. I have found that with alot of hi RPM riding, they will puke oil out the breather. That is why I have moved my breather to the Flywheel cover and closed off the one on the valve cover.
Also remeber it doesnt take but a few drops to make a big spot!

  • MaxAZ

Posted October 05, 2004 - 02:08 PM


I've heard that high RPMs make the problem worse but I don't think I ever rev that hard. I've never hit the limiter and I'm probably no where near it. Maybe my bike just throws oil. My thing has always been that even during a long ride over 3-4 hours, when I stop, I always notice oil under the bike even if it's only been a couple of minutes resting. Makes me wonder how much I'm throwing out all over the trail. Probably enough to give a tree hugger a heart attack.

  • sirthumpalot

Posted October 05, 2004 - 03:48 PM


Do you ever have to add oil between rides?

  • MaxAZ

Posted October 05, 2004 - 06:09 PM


Yes I've had to add oil especially if it's a hard ride like sand dunes. Don't know how much I'd have to add now after the rebuild since I've just been completely changing the oil after every ride lately. I think I'm broken in now to where I'll get 2 or 3 rides on each oil change now though so I'm going to try and get a good measurement on what I'm losing. I probably won't get around to playing with it until this weekend though.

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  • lightestYZF

Posted October 06, 2004 - 07:02 AM


I don't think rings would have an effect on oil through the vent. Rings affect oil consuption via burning out the exhaust from blow by. Somehow you have a surplus amount of oil in the valve cover or it's being sprayed around more than normal.

Oil gets in this valve area primarily in three ways.
1)The oil pump sprays the cams through the external oil line (don't see how this would affect things unless its spray is somehow misdirected).

2)Cam chain flingiing oil off from its trip through the sump (again not sure why this would be different unless more oil in your sump than normal - maybe different type of oils could change this some).

3) Excess oil in frame oil tank draining in through the frame overflow line that runs from left side of frame to valve cover. This is normally the cause.

I would try two things. One check your oil dipstick against another bike. I have seen them to be different lengths. Frames can have a different level from each other so maybe your frame is such that the oil sits slightly higher or your frame overflow is slightly lower than normal.

Bottom line to fix you need to lower the amount of oil you are putting it. Set it so just shows on the dipstick and do a test to see if you get less oil dripping.

Another cause of high level oil in tank would be reduced flow to the oil filter. The frame oil screen could be clogged or that oil line from frame to engine could be bent/crushed. Either would limit oil flow out of frame causing backup in frame that then drains through the over flow to the valve cover.

I hate leaking bikes from my old Triumph days.

  • YZ250F_Rider

Posted October 06, 2004 - 09:29 AM


Rings affect oil consuption via burning out the exhaust from blow by

Not quite, but close.

The oil ring determines oil consumption by whether it successfully scrapes the oil off of the cylinder wall as the piston goes down on the intake stroke.

The compression rings determine the amount of blowby that happens during combustion due to their wear and the wear and carbon build up on the upper part of the cylinder.

The combustion rings can have excessive blow by leading to the misted oil eventually finding it's way out of the vent hose. The oils mists considerably inside of engines from parts hitting it as it drips back to the crankcase.


I didnt see where you said you replaced or replated the cylinder. Without a new cylinder honing for the rings to seat with they may still have considerable blowby. You didnt say how you broke the rings in either. Poor initial wear can lead to lots of blowby since the rings never fully seat.

Look for a very course soft foam, and put it in the vent tube just at the point where the tube attaches to the tube on the valve cover. It will be course enough to let it breathe, but still stop the misting oil from blowin past and then later dripping out of the hose.

  • Davidyz450

Posted October 06, 2004 - 12:09 PM


when you replaced the piston and rings did you have the cylinder bored and/or honed. It sounds like you have excessive blow by. Possible the rings didn't seat correctly.

  • MaxAZ

Posted October 06, 2004 - 04:05 PM


The cylinder was honed prior to reassembly. All work was done by a friend who is a professional bike mechanic with years of experience. He knows his stuff and is highly sought after in the street bike racing arena. I broke it in per his advice by repeatedly warming up the bike to operating temperature and then shutting it down to cool. This was supposed to get the rings seated better and faster. When I did initially ride it, the first couple of rides were very easy and on hard pack terrain. Nothing like inviting some slower riders along to keep you from pushing it. Going back to my initial post, since the rings were not far out of spec anyhow, I doubt the cylinder walls have anything to do with this and it was already blowing out the crank vent prior to the rebuild. It wasn't blowing smoke either but was beginning to lose some power off the bottom. That's why I rebuilt it. Right now, I think it runs great.

  • grayracer513

Posted October 06, 2004 - 06:17 PM


I don't think rings would have an effect on oil through the vent.

Oil rings don't, but compression rings do. Leaking compression rings are the source of combustion "blow by", or combustion gas pressure blowing past the rings, which elevates crankcase pressure, which in turn has to go somewhere. Crankcase air is ideally just supposed to move up and down inside the engine in sync with the piston, so that not much ever really flows out. But blow by at the rings (which all engines have to some small degree)creates a source of "new air" that flows out the vent in a more continuous manner, carrying oil with it. If the drops on the floor are black, instead of looking like cleaner oil from a leak somewhere, they are the result of a thoroughly slimed vent tube dripping dry.

  • FFRacing79

Posted October 06, 2004 - 09:51 PM


1)The oil pump sprays the cams through the external oil line (don't see how this would affect things unless its spray is somehow misdirected).
***The cams are not "sprayed" with oil, but rather pressure fed thru passages in the journals(fed by the external oil line).***

2)Cam chain flingiing oil off from its trip through the sump (again not sure why this would be different unless more oil in your sump than normal - maybe different type of oils could change this some).
***The cam chain does not go thru the sump. If the oil level is high enough to touch the chain while the engine is running, you have a big problem****

3) Excess oil in frame oil tank draining in through the frame overflow line that runs from left side of frame to valve cover.
***The line you are referring to is a tank breather line, not an overflow line***

  • YZ250F_Rider

Posted October 07, 2004 - 03:17 AM


well there is one thing you can do to see if it's blow by or not.

get a valve stem from a bike or a car, and trim the round bulb end so it fits nice and tight in the tube coming out of the valve cover. Run the bike for a while and see if it builds up pressure.

You dont want to let it get more than a pound or so, since the rubber hoase running from the top of the frame back to the valve cover isnt clamped tight enough to take much pressure.

  • sirthumpalot

Posted October 07, 2004 - 03:38 AM


Just an FYI; I believe these things have plated cylinders and I'm not sure if honing is recommended. Can anyone back me up on this?

  • YZ250F_Rider

Posted October 07, 2004 - 05:59 AM


you have to hone the nikasil plated cylinder with a diamond. But yes they are honed.

  • MaxAZ

Posted October 11, 2004 - 06:18 PM


Seems like a strange test but I'll give it a try. I might be able to actually use some sort of pressure or vacuum guage instead though so I can actually read what's there.

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