R100CS throttle issue

Tridentested

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Hi All
I have a '81 R100CS, 67,000 miles, generally good condition, starts and runs well no issues - apart from:

Starts fine on the choke and soon off choke. It revs cleanly all the way and settles back to an even tickover. However once hot if you rev it over about 2600rpm and shut the throttle it only returns to about 2000 rpm. Rev it below that figure and it returns to a normal tick over. Put it in gear and let the clutch out a bit and the high revs return to a normal tickover. So sounds like a sticking throttle? I've changed the throttle and choke cables for new, checked the cable run, made sure the throttle friction control is at its loosest, checked the carb spindles run smoothly in their bores, changed the diaphragms for new, all to no avail.
So I'm stumped - anyone out there got any ideas I can try.

Cheers

Millerboy
 
If you have ruled out the carbs by properly cleaning all the jets, setting idle mix, renewing diaphragms etc, I would next be checking the timing advance.

Did you set your idle and balance initally with a cold or warm engine?
 
gog said:
If you have ruled out the carbs by properly cleaning all the jets, setting idle mix, renewing diaphragms etc, I would next be checking the timing advance.

Did you set your idle and balance initally with a cold or warm engine?
Click to expand...

Hi

Other forums seem to point at weak return springs for the timing so will check that out first. Thanks
 
millerboy said:
Hi

Other forums seem to point at weak return springs for the timing so will check that out first. Thanks
Click to expand...

I wouldnt mess with the timing until you have absolutely ruled out the carbs being incorrectly adjusted, including throttle and enrichener cables.
 
scooter paul said:
High tickover can be an air leak. Check carb rubber manifold.
Click to expand...

he can get it to return to 'normal' tickover with a blip of throttle, I would expect if it was an air leak it would be continuous, as it probably would be with incorrect adjustment or dirty jets. sounds like something is sticking, advance or a cable. could be totally wrong though.
 
FWIW I get this sometimes on my R90/6 when the choke isn't returned fully. Sometimes I knock it with my foot, especially if I've used the kickstarter.
 
Check the idle mixture screw settings.
Manual may say 1 1/4 turns out from fully in.
If you are happy that the carbs are ok,take them in a further 1/4 turn and road test.
You can always put them back to where they were if it changes nothing
 
Re what Gelandestrasse said. Could it be that while the choke lever is fully closed the lever on the carb moved v slightly up the cable when the small nut was being tightened. Could be worn jets, needle or O rings, which may not have been changed in years.
Failing that try and get hold of a copy of BMW Club SW Section Tech Bulletin No3 by Mike Fishwick on Bings.
If you can't locate on I can scan mine and send you a copy
 
digger37 said:
BMW Club SW Section Tech Bulletin No3 by Mike Fishwick on Bings.
Click to expand...

My Bing Bible, too! I rate his gearbox guide, too.

Forgot that the choke lever is probably by the clutch lever on this model. However, if the choke cables don't return properly every time, you will have a high tickover.
 
Hi Everyone

Thanks for all your suggestions. All sensible about carb adjustments and air leaks but once hot it will start on the button and tick over as it should and rev as it should, only doing the 'sticking' after revving higher, hence my concentration on cables and diaphragms, but to no avail. Have just taken off the front alternator cover and confirmed that although fitted with electronic ignition as per an 81 model there is a spring operated advance retard you can move with your fingers. However the springs seem strong and the unit snaps back freely when you let go?

So I think the jury is still out!
 
I experienced this problem on my mates R100CS and my own R80GS after rebuilding the carburetors. The redemption I found was to lower the idle speed, with hot engine, from ca1250rpm to 1100rpm. This eliminated the problem. I have no idea what causes this, as other airheads seems to have no problem idling at 1250rpm.. I don't believe it has to do with the ignition timing advance as this was both checked and tried with another beancan on my R80GS. Also both throttle cables were swapped for new on both bikes - one with the Venhill cables another with the original BMW cables which goes a lot more smooth.
 
JakobDK said:
I experienced this problem on my mates R100CS and my own R80GS after rebuilding the carburetors. The redemption I found was to lower the idle speed, with hot engine, from ca1250rpm to 1100rpm. This eliminated the problem. I have no idea what causes this, as other airheads seems to have no problem idling at 1250rpm.. I don't believe it has to do with the ignition timing advance as this was both checked and tried with another beancan on my R80GS. Also both throttle cables were swapped for new on both bikes - one with the Venhill cables another with the original BMW cables which goes a lot more smooth.
Click to expand...

Wow, that’s a high idle, all my airheads idle at 750-850rpm
 
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