I'm reassembling a 1985 K100RS that came to me in boxes.  The drivetrain was substantially intact, but the clutch cable had been removed so the swing arm was just hanging and the boot between the swing arm and the push rod was torn.  I replace the boot, reinstalled the cable, and adjusted per manual spec.  But actuating the clutch lever doesn't seem to engage the clutch at all.  Just to test, I ran the adjustment screw on the back of the arm all the way in to maximize the throw on the push rod.  Still no engagement of the clutch.  What are possible causes?

Thanks in advance for your help.

G'day, this may be a daft idea but here goes. Is the clutch friction plate fitted the correct way around? Is the release bearing fitted correctly.? Is the whole clutch assembly assembled correctly? Is the clutch cable the correct one? Sorry these suggestions may seem stupid BUT????? Cheers Tom

No suggestions are stupid as far as I'm concerned.  I don't think the clutch was ever disassembled.  Just the cable was removed causing the torn boot.  I suppose something under boot could have slipped out of place.  I know the spring is in there, but I really didn't looked at what was behind the boot very carefully.  I think I remember seeing the bearing.  And the adjustment screw does contact something when it's threaded in part way.
I'm pretty sure the cable is correct, although I plan to replace it with a new one.

dnkn wrote:. . . actuating the clutch lever doesn't seem to engage the clutch at all.  Just to test, I ran the adjustment screw on the back of the arm all the way in to maximize the throw on the push rod.  Still no engagement of the clutch. 

Were the engine and transmission attached to each other when you received this relic? Which is happening now—is it seized in gear, or seem stuck in neutral?

Engine and transmission were attached.  It isn't stuck in gear or in neutral.  I can shift the gears.  If the bike is running, it clunks into first and the back wheel starts spinning, as if I didn't pull the clutch at all.

Hi remove pushrod inspect for damage, 2 the clutch plates maybe frozen. good luck

Push the rear brake on to see if the wheel or engine stops

moriarti wrote:the clutch plates maybe frozen. good luck

+1  This is where I would start. 

What is the condition of this bike relative to corrosion?  Does it look like it has spent time outdoors?

If the plates are stuck, the suggestion of pulling the clutch in first gear and then hitting the rear brake may break them loose.

The bike was stored inside, so it's very clean, no corrosion at all. But I'll try the rear break and take a look at the push rod. Thanks for the suggestions.

I haven't time to try the suggestions made so far, but a friend of mine suggested that if the clutch plates sat without lubrication for decades, they may have frozen together.  Is that a plausible theory?

Clutch plates in a K don't have any lubrication they are a dry clutch. The clutch plate may have rusted to the pressure plate causing it to bind.

The environment of the Brick clutch assembly is not hermetic so humidity and condensation can occur within that confined space regardless of whether the moto is indoors, eventually leading to the plates and fibrous disc seizing together as RickG, moriarti, and Point-Seven-five have indicated.

The clutch assembly, clutch piston, clutch bearing and clutch rod all must be inspected to determine their condition. The transmission input shaft splines would need thorough cleaning and lubrication after a period of neglect anyway, so it's fortunate that this condition occurred before damage of unlubricated shaft or clutch hub splines left you stranded in the wilderness—urban or otherwise. 

You should post well-lit, high-res photos of those splines after they've been cleaned but before they they've been lubricated so their condition can be viewed here.

When you use the word "engage" what do you mean?

When a clutch is engaged that means that a clutch is transferring power from the motor to the transmission and drive train. When you pull in the clutch lever that should disengage the clutch.

Engaging the clutch allows power to transfer from the engine to the transmission and drive wheels. Disengaging the clutch stops the power transfer and allows the engine to continue turning without force to the drive wheels.

https://www.edmunds.com/car-technology/when-fact-meets-friction-the-basics-of-clutch-operation.html#:~:text=Engaging%20the%20clutch%20allows%20power,force%20to%20the%20drive%20wheels.

In that case, the clutch with not disengage.

On a bike that came in boxes of parts with a "mystery history" you're going to want to pull the  transmission to inspect/lube the splines and assess the clutch assembly anyhow.  I'd get to that first and not waste any more time and effort before on this you do that.

I've dealt with oodles of old K bikes, either to part them or bring them back from the dead, and that would be what I'd do.

JM.02

YMMV

I had a similar issue when recomissioning my '86 RS after it had been stored unused in a garage for 12 years. The clutch plate had become stuck to the flywheel over that time and I freed it by putting bike in 4th or 5th gear, with clutch lever activated and rocking it back in forth without the engine running. This is a quick and easy thing to do and I dont think should cause any damage.

Iain