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I have only had the '90 for a couple of months and the odometer cable broke at 3150 miles, so I don't know the total number of miles on the bike. After riding for an hour or two of tight woods, open fields 6th gear blasts and some 30mph street riding to get back to the house, I get transmission oil dripping out of the air box. Tranny is NOT overfull. The first time it happened I thought it was because I had dropped the bike on the right side and some leaked out before I could pick the bike back up, but that is not the case as it happens every ride. I know it is coming from the trans vent tube because you can see where it drips on the air filter and the spray pattern on the inside of the air box. I have checked the vent tube at idle and revving on it in the garage and there is no pressure coming out of the vent tube. I am wondering if there is too much vacuum in the air box(unmodified with only the snorkel removed) at high rpm that is sucking the oil out of the vent tube. I am thinking of making a catch can out of a Mighty Vac brake bleeder bottle and removing the vent tube from the air box all together. Anyone have a similar experience?

David

HMMM I would have done the same as you to eliminate a case seam pressurization leak. I have an 89 and the airbox lid is totally off. I don't have this problem. I would do as you say and remove the tube but put it somewhere where you can see it like at the front of the tank or something and go for a short ride where you load the engine and turn etc. Might give you a better idea of the cause

Does the transmission get really hot? How long does it take to loose how much oil? Is the transmission getting low as the oil goes to the air box or does the level stay about the same?

I haven't checked the temp, but it doesn't smell hot and I don't notice it on my ankle. It is about .5 ounce in a couple of hours. It is getting lower in the sight glass but it is still visible, I have ridden it 3 or 4 times that I have noticed the oil after parking it.

If you're sure the trans is not overfull, I'd be willing to bet you have a main seal leak. Combustion pressure is leaking out and pressurizing the transmission case, causing it to piss out fluid.

I hope not, but the seals are old and getting hard. Is that one replaceable by removing the clutch?

No, sorry but that will require splitting the cases.

If you do a search on here there is a discussion about checking for that.

Everything I have read about a leaking right side crank seal says it will suck tranny fluid into the crankcase, not blow it out of the transmission which make the most sense as the pressure should be minimal as the piston is coming down and the new mixture is being push up and around to the combustion chamber. Vacuum, created by the piston going up that is used to pull the new air/fuel mix in from the carb, should be much greater than any pressure on the underside of the piston. I don't know the answer yet. Still working on it. I have removed the vent line from the air box and now have it hooked to a Mighty Vac brake bleeder bottle. It fits nicely on the right side of the carb by the brake reservoir. Now I just will have to wait for the weather to clear up to test it out.

david wrote:as the piston is coming down and the new mixture is being push up and around to the combustion chamber.

The piston coming down won't make pressure once the rings pass the ports? What causes the mixture to be pushed up and around to the combustion chamber?:hmm:

A bad crank seal typically does not make pressure in the crankcase but it can happen.

Good luck with the search for the problem.

A leaking crank seal can leak in either or both directions.