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Final drive vent

Is it possible that you have recently changed the brand or type of oil in the rear drive?
Did the venting problem start with this trip having been ok in the past or is this a new bike to you?

You say you used 80w90 gear oil - that is correct as I understand but the reason for my asking above is
that once in the past on an earlier year airhead I thought I could slow or cure a leaking drive seal problem
by using 85/140 gear oil instead of the 80/90 - the result was that the thicker oil was thrown up to and out
the vent like the problem you describe - my experiment failed and made a mess

The type or brand of the oil you used may be an issue if you have changed just prior to this problem.
 
Same oil since i bought the bike 5 yrs ago, 20k miles, valvoline durablend part synthetic, 80w90. The vent has always done this, the more close to full the worse the blow by through the vent which is also tied directly to speed. I could have it full and ride around all day at 40 mph and just get a little oily wetness around the vent cap. If the oil level is 1/2 or more below the 9:00 bolt hole i could ride all day at 70 mph and see the same. If it's full and i ride at 70 it wants to blow oil all over. Could someone post a picture of the inside of their vent cap? I'd like to compare it to what i had. I've welded a baffle in mine already otherwise i'd post a picture of mine.

Thanks,
Jim
 
same thing happened to my '74 900. Conditions, cool temps, just changed from BMW gear oil to Dino Valvoline 80/90, went for short spin, came back foamy oil all over the place oozing out breather top. Panic. What I remember is advice concerning how at a times, when conditions are right, the gears can shear air bubbles into the mix creating greater volume then housing contains, there's nowhere for the frothy mix to go except out the top. There was a thread explaining all this back in the lost threads somewhere, around aught eleven. I now use BMW gear oil with a pinch or two of moly. Inner seal has just started a infinitesimal leak, another project. Two cents FWIW.
 
I believe there is a baffle inside

I checked and resealed a finned or '80 and up F/D similar to the OP and found a sheet metal baffle plate inside that appeared to be there to direct the oil splash away from the vent. Mine has never pushed oil out the vent.
 
Not if you can see the gears, but I checked one earlier this morning and it (an '82 FD that did not have a baffle) certainly accepted a baffle from a later FD. Maybe some of them are just missing?
 
Went out for a ride today. Vent cap puked oil all over the place. I called max bmw and talked to rusty about my issue. I took off the rear wheel and the oil was not coming from any of the seal. Rusty said there is supposed to be a baffle inside the final drive and if i can see the crown gear rotating mine is missing. I'm going to hook up a puke line with a banjo fitting while i wait for the baffle to arrive from Germany. Why it has never done this before i have no idea.
I did check the fluid levels when i returned home. Driveshaft was right on, final drive was 100 cc low. Ride was about 80 miles.
 
I just checked my 81 R100RT and it has a baffle! Where is it listed on the parts diagram?
81FDbaffle_zps9cc0c118.jpg
 
How much different is the pre 1981 model final drives that they didn't require this "protection plate"? The fiche says it began use in 9/1980. Why don't the earlier FDs spew oil??
 
Different casting and vent

I would have to say its the shape of the casting and the vent itself is built differently. The problem with making a puke line for it is you can run the gear oil right out of the F/D on a long ride and damage it. I would have to guess that the F/D's out there without the baffle were resealed at one point and the wrench that did the work got it back together then realized the baffle plate was still on the bench. Didn't want to waste a gasket so the baffle got tossed. There's a ton of those late model F/D's out there so I'm sure someone will have a spare one you pick up. Takes about three hours and $25 in parts to pull one apart and reseal it if you don't have to re-shim or put new bearings in. Mark Hugget may have those baffles, he seems to have a lot of N/L/A stuff.
 
Apparently my final drive doubles as a hydraulic pump. Took it out this morning. Went about 10 miles no faster than 45 mph. No oil in the vent tube i installed. Hit the highway and did about 4 miles at 70-75 mph and this picture was the result. Got back home and nearly all the oil had drained back into the FD. Lost no oil and the rear tire stayed dry. Thanks for all of the help.
 

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Apparently my final drive doubles as a hydraulic pump. Took it out this morning. Went about 10 miles no faster than 45 mph. No oil in the vent tube i installed. Hit the highway and did about 4 miles at 70-75 mph and this picture was the result. Got back home and nearly all the oil had drained back into the FD. Lost no oil and the rear tire stayed dry. Thanks for all of the help.
Are you going to do a proper repair with a baffle?
 
Yes, but probably not until the riding season is over and the bike is in my basement. This fix just keeps me riding for now.
 
One thing that probably isn't helping your situation any is that it appeared that the cap of your vent was shoved all the way down to the top of the threaded nut insert in the final drive. It should be up above the top surface of the nut by a few millimeters so that it is open to normal atmospheric pressure through the tiny holes at the top under the mushroom cap. Otherwise, it may be that there is no air flow/venting going on, and as the final drive heats up under high speed riding, things expand (including the air inside the drive) and it forces things to squirt out under pressure. I have seen this happen on final drives where the oil is forced out of various seals in the final drive, coating the rear wheel in FD gear oil. Simply making sure that the little vent holes in the top of the cap were not plugged, and ensuring that the vent was not shoved all the way into the sleeve of the final drive (it should look like a "mushroom" with a couple mm of "stem" showing ) and it fixed the problem.
 
The vent was operating properly. Must just be the angle of the picture that makes it look like you described.
Thanks,
Jim
 
Finally got around to opening up the rear drive and sure enough, no baffle. Took a few weeks for the part to arrive from Germany but it is in there now. I'll leave my sump line attached until i take it out for a spin this spring and confirm that this was indeed the issue.
 

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