foaming rear drive breather....

[8ninety8]

I tried a search but it comes up blank, so, after a short spin to warm up R90S, got back and surprize, breather oozing well foamed gear oil out the top!

Don't know how such a short spin in second gear to warm up carbs could so quickly aerate the gear oil. Never happened before, seals?

One thing, I just remembered, I was running without the air tubes connected to the carbs, maybe this caused some kind of pressure that normally is sucked into the right intake from the crankcase breather?

When I took the filler plug out the oil was well foamed, I mean FOAMY. Full of little bubbles. Just can't figure how running with the crank breather open to atmosphere could increase pressure through the trans, into the driveshaft?, and into the rear drive? Can't be.

It's got regular gear oil, GL-5, 80-90W.

[ebeeby]

Um, you didn't by chance just get your bike out of storage and give it a good wash - maybe with a pressure washer?

[8ninety8]

I used, however, Valvoline gear oil, I think I'll go get some BMW gear oil tomorrow. I think I really filled it up last fall, you know, right up to the threads. I never seen this before. Foamy? gear oil?

I only hopped around a few blocks, maybe 15 minutes in second gear to warm everything up. Oil is clean, full of little bubbles. I know there is a simple explanation, but I can't find it in other threads.

[PMonk]

Happened to me once back in the 70's.

Seal went out between driveshaft and final drive and was just enough to
send oil out the breather.

What weight are you using and is it possible you got water in?

[8ninety8]

It was a fresh change last fall, guess I'll take it apart since I haven't been in there for many years. This bike has always been in the garage, covered over the winter. The good is that it idles perfectly after I tweeked the old Volt regulator. Win one, lose one. See it on p. 55 under pics.

[MotorradMike]

This happened to me with my Yamaha(shaft drive). I asked the dealer about it and he said "Put synthetic in, that usually stops it".

He was right.

[PGlaves]

Foaming is usually caused by some moisture in the oil. Sometimes, when it is cold ambient air temperature the ring gear simply chops air into the oil, but normally moisture is involved.

I would simply drain the final drive oil and replace it with a fresh fill of a quality GL5 rated 80w90 gear oil. Castrol Hypoy C is my preferred brand but I can't say that other brands with the GL5 rating and 80w90 viscosity won't do the job required.