Hello again,

My '89 100RS 16V is having some problems again. This time, it's the clutch.  

I just renewed the clutch friction plate, and the spring plate, because the old one would slip under high load. (was good for mileage however )
I reassembled the lot, took off for a test drive, and I was amazed by how supple the clutch worked. After a few miles I got to the highway, where I rode 40 miles in 5th gear, not using the clutch. On the exit, in order to change to 4th, I grabbed the clutch, but then it felt really heavy. Pulled it in, changed gears, released the lever: nothing. It just felt "stuck" in the depressed position. I looked at the lever arm, and that's moving freely. After about 15 minutes of tapping the loose hanging lever (the one on the handlebar), it suddenly jerked back in it's good working state.

This scenario repeated itself a few times on the way back. Only on highways though. (scary!)


When renewing the clutch I saw the pushrod-piston (with the bearing) was really rough. I sanded it smooth again, lubricated it, but I guess it sticks anyway. No idea why however 

I'll try renewing this piston, and let you guys know if it helped, but if anyone has an idea of something else that might be causing this, please enlighten me!

That sounds like a cable problem to me have you checked it for smooth operation.

Ah, Yes, that was my first thought too.
But, unfortunately, after replacing the cable assembly, and double checking that it works without resistance, the problem remained the same...

Check the pivot bearings on the clutch operating arm at the rear of the gearbox when you disconect the cable it should swing very freely. There are a couple of roller races in there that get rusted up very easily. There is a current thread on here about having to cut the shaft with a grinder so as to get it out.

I'm with Rick. If the actuating arm at the gearbox is working correctly and the handlebar lever is 'dead' then it must logically be the cable.

The cable may be fine but the routing of the cable might be the issue. If it's too acutely turned somewhere that could cause it to hang-up for a while before releasing after a short time.

(I think he says in the OP that the actuating arm is free)

Yeah, the actuating arm is free indeed, and so is the cable.
When the clutch sticks, I can pull the handlebar-lever freely, and I can see that the pivot-(actuating-)arm follows neatly, w/o much resistance.

Logically, the next link in the chain is the pushbearing piston. nr. 3 in this view.

http://www.realoem.com/bmw/showparts.do?model=0523&mospid=51708&btnr=21_0252&hg=21&fg=51

Either that or the pushrod possibly is bent. Easy to do when removing the gearbox, I've done it.
Best to remove the pushrod out the rear of the gearbox before you remove the box itself unless it's a K75 then you have to leave it there as it only comes out the front.

I think we need to clear-up the symptoms.

When the clutch 'sticks' does the handlebar lever return or remain against the grip?

If it does return, is there still the same pressure needed to pull the lever back to bar?

Or is it that the clutch itself is not engaging - the handlebar lever returns, the actuating arm returns but no drive goes to the gearbox?

@MikeP,

The handlebar-lever does return, but only due to its own weight. Thus it returns, but you can pull it back again with no resistance (even blowing against is would be enough).

@RicK G

The bar is dead straight, so unless we are missing something, I guess it must be the "piston".
I'm looking for a new one. I'll report if that solves the problem.


Thanks a lot for a valuable second opinion, guys!

i´ve had sort of the same problem once , it turned out to be a litlle stone lying on the exaust  just able to sometimes block the clutch arm  , and sometimes not, this however only lasted for half an our  or so ,before i located the problem and removed the stone   
just a thougt,
k-rider

I doesn't leave much else except maybe the diaphragm spring but I cant imagine how that would give that sort of problem. I think it would have to turn inside out to do that.
I hope for your sake the piston hasn't damaged the bore on the gearbox as that means a replacement case at the least.

Fixtit!

As I guessed, the part that seemed to be the problem was the plastic housing containing the bearing pushing the rod. That's the round part that just behind the rubber gaiter.
It still looked quiet good, but it felt a bit grooved, as if some sand had gotten behind the gaiter. I got one for free from the guy that sold me the gearbox, put it in mine, and away flew my sorrows! Hurray!

Ready for another 100.000km's.... Well almost