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R100RT. Does anyone know why the idle speed drops when I pull the clutch lever (doesn't matter whether it's in gear or not)?
I changed the clutch thrust bearing thinking its the only thing that could be responsible (no difference), and checked the end float on the crankshaft - no discernable movement. There are no odd noises and everything works as it should.
I have only noticed this after a significant overhaul (clutch, big ends and carbs have all been apart). I say this because I'm not ruling out the possibility that I reassembled something wrong, however I think the effect was present before but I never made the connection with the clutch because it used to hunt badly anyway.

Sticky/sticking ATU?

First thing to come to my mind is clutch cable snagging carby cables under the tank, or further forward near the steering head?

Edit, it won't take much movement of a cable/s to cause a problem.

How much drop, like 100-200 rpm?

Mine has always done that, I have always thought it was normal for the bike.

Thought I had read that or been told that....

Mine has never done that. '74 90/6

You might want to start a thread and find out what's wrong with your bike.

Nice bike choice though!

Isn't this an issue with the end float on the crankshaft? When the clutch is pulled, you put pressure on the flywheel which results in a change in drag on the crank. With the engine off, does the flywheel move perceptibly when viewed through the timing hole when the clutch lever is engaged?

Kurt in S.A.

Kurt in S.A. wrote:Isn't this an issue with the end float on the crankshaft? When the clutch is pulled, you put pressure on the flywheel which results in a change in drag on the crank. With the engine off, does the flywheel move perceptibly when viewed through the timing hole when the clutch lever is engaged?

Kurt in S.A.

Thats the classic source of this problem. The cause is wear in the crankshaft thrust bearings which are located either side of the rear main bearing... a full strip down to change ... not a 5 minute job I'm afraid.

Provided it isn't too bad, it's a nuisance rather than a show stopper.

Rob

Many thanks for the quick replies guys. My first thought was the ATU - it's fine. I hadn't considered the cables, but I checked last night - not that. I too wondered if it was normal behaviour but thought that would be uncharacteristic for BMW. From your responses I guess they don't all do it (and yes the drop is 100 - 200rpm).
I have tried checking for movement by feeling the alternator when pulling the clutch - nothing discernable. I can feel increased friction when rotating the crank by hand with the clutch pulled, but no more than I might expect from the load on the thrust washer without oil pressure. My instinct is that for friction to shave 200rpm off the idle speed, a fair amount of energy would be absorbed, and I would be able hear/see/feel the result. Maybe I'm wrong?

Rob Frankham wrote:

Kurt in S.A. wrote:Isn't this an issue with the end float on the crankshaft? When the clutch is pulled, you put pressure on the flywheel which results in a change in drag on the crank. With the engine off, does the flywheel move perceptibly when viewed through the timing hole when the clutch lever is engaged?

Kurt in S.A.

Thats the classic source of this problem. The cause is wear in the crankshaft thrust bearings which are located either side of the rear main bearing... a full strip down to change ... not a 5 minute job I'm afraid.

Provided it isn't too bad, it's a nuisance rather than a show stopper.

Rob

The drag can make that much difference? Surprised.

With the VWs that make most of my motor experience, a lotta endplay will advance the ignition when the clutch is disengaged. Idle speed will pick up. Definitely not a factor on an Airhead.