Vintage BMW Motorcycle Owners

If you pull the cover on the back of the transmission, a thorough job of reshimming...or checking the shimming...of the internals needs to happen. It requires special places and the ability to precisely measure clearances. This is not an easy thing to do in my opinion.

That’s what I was afraid of.

Clean the area around the shifter lever seal and check it for leaks. As for the back of the transmission, the boot on the clutch lever could be bad and leaking or, most likely it is the neutral safety switch leaking. It is pretty rare the back cover gasket will leak.

There is a way to replace the neutral light without pulling the transmission, sorry I have not done it in years so I don't remember exactly what has to be taken apart to do the job. I do know doing it this way requires far less disassembly. Do a net search and the procedure may turn up. St.

Snowbum discusses the neutral switch and installation at the bottom of this page.

https://bmwmotorcycletech.info/neutralswitch.htm

The key is to bevel the ends of the support tube and put it in the freezer to shrink it. Then it should slip up and in place when ready.

I don’t think it’s the neutral safety switch. It appears dry around that. I do think the shifter shaft seal is leaking a little as it’s damp right around the shaft. I was also looking at the clutch lever assembly as this looks like a GREAT place to have a leak, and it could easily run down the rear cover. I’m going to do some more cleaning in the area and run it some more to see if the leak will make itself known. I suspect I’ll end up with quite a few seal problems since this thing sat for twenty plus years. I’ll climb back under it in the next few days and report my findings.
Thanks,
Chris

A dusting of baby powder will easily highlight the trail of seepage and where it's coming from.

Seals that sat for 20 years are dried out and probably all failing or preparing to. I can't speak to the R90S gearbox but I just recently brought a 1984 R80 G/S back from a long nap and her motor and gearbox were totally caked in dried old oil/dirt/sludge you name it. Before I ever ran the bike I tore the motor out and the gearbox and replaced every seal in both. That didn't require me to tear down the gearbox though I did need to source a few tools to install the rear main seal in the motor and I needed a few tools to pull the output flange off the gearbox to replace the output seal. I ended up being able to replace all the gearbox seals without removing the rear cover of course while it was on my bench it was very easy to install a new neutral switch etc. I had never done essentially any of these jobs prior to this and I'm happy to report that after 600 miles on the road she hasn't leaked a single drop. In my opinion it's just easier to do it all right from the jump than continuously chase your tail with issues for years.

When I go through a machine that has sat for quite a few years to replace seals, etc. without doing a full teardown, before I remove the old seals, I fill both the engine and trans, rear end, etc. with either kerosene or gas (outside of course), let them soak and then shake them as best I can every hour or two, drain and repeat a few times until the liquid comes out clear. You'll be amazed at what comes out. Use some compressed air to vent things until there's no more odor and then start the seal replacement.
Hope that helps.

Thanks for the ideas, guys. To be honest, I already have two projects for the winter, so yanking the engine and transmission out of an R90 might have to wait. I’m a little afraid to pull the bike too far apart because I have a disease called “Since you already have the bike torn down you might as well restore it and make it perfect” syndrome. It happens to me every time I acquire an old bike, and as you guys know, it gets expensive and takes hundreds of hours. I agree that replacing every seal right off the bat would be best, and I might just do that, but I’ll have to see how my other two winter projects go before I tear into it.
Let me ask this question while I’m here…
I’ve read that the 74 transmissions are a weak link on these bikes and a lot of people replace them with a later box. Are there really later transmissions laying around just waiting to be bought? Is there something you can do to beef up the 74 transmission to make it better/stronger? Or, truthfully, are the 74 transmissions really that bad? I seem to find an occasional false neutral between 4th and 5th gears, but it’s not too bad. Maybe once every 50 miles or so I’ll find it. Other than that it seems fine. The bike has roughly 28K on it. If that means anything.
Thanks again, guys!
Chris

I suppose it is hit or miss with the '74 transmissions, but it was the first year so one can understand the "growing pains". Snowbum discusses model differences on this page. There is a section about 1/4 the way down that discusses the /6 and he has a paragraph with his impression of the first 5-speed transmissions:

https://bmwmotorcycletech.info/models.htm