Interesting. I don’t know about your ‘87, so I’m speaking from a ‘78.  I would rock the bike for and aft and get it shifted into 5th. Pull the clutch, and rock it with a bit of force and see if it will break free.  If not, I’d go back to 1st gear, I would kill the ignition with the kill switch, ignition on, foot on rear brake, clutch pulled, and bump the starter to see if it will break free. The bike will want to lurch forward if the clutch doesn’t let go, so caution would be required.  Do not try this at home.

Thats all I can think of.  Someone else probably has better ideas  

I should have prefaced my comments by saying - be sure the clutch is properly adjusted and actuating as it should FIRST. 

Big question is why. Did you lube the splines when is was apart?

@4949 Thanks for the suggestions.  The first time it stuck, I tried the 5th gear rocking with the clutch lever pulled in.  It just turns the motor over. 

I've also tried riding the bike around with the clutch pulled in.  (A bit tricky to get started.) I did this on my property, not on the road, so I was limited how far I could go, and could only get up to 3rd gear.  I tried accelerating then hitting the brake several times but it didn't break free.

Yes, I lubed the splines with Honda 60 Moly the first time I had it apart.  Not sure what impact that would have on clutch sticking.  I suspect that the reason the clutch stuck was due to being stored in a garage with high humidity, and not getting ridden much.  I've tried to remedy that by putting a dehumidifier in the garage, but it stuck again.  If I have to pull it apart again I may pop for a new clutch disc.  I have 9 other airheads in the same garage and none of them have stuck.  Perhaps the clutch disc in this bike is made of a different material.  

Dale

That is quite a conundrum.

I wanted to rescind my suggestion of bumping it with the starter. Thinking about it, I got concerned about damaging the flywheel teeth or starter. 

With a spline lube issue, I could see the clutch being reluctant to release, but yours is frozen solid. Sounds serious.

My clutch (pressure ring, pressure plate, and friction disk) for my ‘78 is on it's way back to me from Southland Clutch. This round I chose to have the clutch refurbished instead of buying all new parts. I did buy a new spring, of course. Anxious to experience the results. Mine was beginning to slip in top gear and full throttle. 

@4949 Thanks, David.  I should think your clutch should perform perfectly after a refresh.  Did you determine the cause of slippage?  Was there any evidence of oil leakage into your clutch cavity?

I copied this from Snowbum’s material, regarding unsticking a stuck clutch;

I prefer to try the following things in order shown below; either method of the two shown can take up to 6 tries.
I have listed TWO methods, use either one, if one does not work, then try the other method.

Method #1:
(a) Put the bike on the center-stand; securely jack the bike at the rear frame crossover (swing arm area?), ....be sure you have the rear wheel definitely off the floor surface.  Hold the front brake!  Start the engine with the transmission in neutral.  The idea is to have the engine a bit warmed up, so it starts more easily again ....because, the next step is to start the engine while the transmission is in first gear. 
(b)  Place the transmission into 1st gear.  Restart the engine.   Do not touch the clutch lever.  At some goodly moderate rpm, the rear wheel nicely spinning, suddenly snap the throttle off (keep that front brake ON!); and, at the same time you snap the throttle off, give very hard pressure with your right foot on the rear brake.  That may force the clutch to release. Try up to 6 times.

Method #2:
Warm up the engine a bit, so it will re-start easily.   Push the bike to the street.  Start the bike in 1st gear.  Depending on model and situation, you may have to pull in the clutch lever at the bars to close a switch which allows the starter motor to crank the engine to restart the bike.  Ride the bike down the street at 4000 or more rpm in first gear.  You can try holding the clutch lever at the bars full backwards during all this, or, at a convenient & safe time, suddenly pull in the clutch lever at the bars ....but, the idea is that when the lever is pulled backwards, you will engage the front and rear brakes very hard.  Try this up to six times.  You can also try using only the rear brake, but, in any/every case.... be careful, you do not want to loose control, slide out, etc.

IF the clutch begins to work properly, I suggest you add some to your short ride, and go through the gears, and use of the clutch, a number of times, before returning home.  When home, do check for proper adjustment on the clutch lever at the bars and at the transmission.  The bars clutch lever cable adjustment sets the transmission lever position; and the transmission lever adjustment sets the bars lever free play amount.   Do the adjustments exactly how it is outlined in this article, above.

If no luck at all, you will have to remove the transmission, & work on the clutch ...or, try prying at the clutch through the timing hole ...which I have suggested you do not, but if carefully done MIGHT work, a bit here and there, as you rotate the engine slowly a bit at a time.

Since this has occurred several times, there's no evidence that it will change its course in the future. A) The friction plate almost has to be of a poor material, or B) oil from either the engine or gearbox has to be getting onto the friction plate.

You still have some winter remaining, I'd go ahead and plan on a friction plate replacement and simply include all the seals and o-rings that could possibly be leaking. BMW went through several generations of oil seals in the late 70's/ early 80's. And too, after 25-30 years, most o-rings and oil seals have grown hard and unable to seal. For a quick check, just go look at the "weep hole" on the very bottom of the bell housing. 

If you don't feel up to this work, then this is the very best time to take your bike to a local shop. Once March hits, everybody and his brother will be digging out motorcycles to ride and the shops will be slammed until late June.

Hope this helps.

Thanks for forwarding the info, David.  Good advice, Richard.  When it was apart last summer, everything was good and dry inside, so I don't believe oil fouling is the culprit.  If there was oil on the clutch disc, wouldn't it cause slippage instead of sticking?  

I have another project in the garage to finish.  Once it's completed, I'll pull the RT in and spend another Saturday disassembling, inspecting, and unsticking. 

Dale

@dale-aberegg be sure to let us know what you find. Can’t imagine what would be making it stick SO WELL. You couldn’t glue it together that good!

Posted by: @dale-aberegg

When it was apart last summer, everything was good and dry inside, so I don't believe oil fouling is the culprit. 

What does the weep hole say ? Nothing told us about the previous work. 

If there was oil on the clutch disc, wouldn't it cause slippage instead of sticking?  

That might depend on the type of oil.

At a minimum, I would try another brand of friction clutch plate while the clutch is apart this next time.