Looking for some guidance on my BMW R90S. I’ve been chasing an idle issue that seems to be getting more inconsistent, and I’m starting to wonder if I’ve got more than just a tuning problem.
Background:
Originally (before any clutch work), the bike was idling a bit low. I tried adjusting it, but had a hard time getting it to hold around ~1,000 RPM consistently. Once fully warmed up, I could sometimes get it close, but it would dip and stumble, and I’d see the generator light flicker like it wanted to stall. I often had to give it a little throttle to keep it alive.
Clutch repair:
I recently replaced the clutch cable. That’s now working great, no issues there.
After that, I thought I had the idle somewhat sorted around ~1,000 RPM.
New behavior:
On a recent ride, I noticed the idle climbing higher than expected, about 2000rpms. When I got back and put it on the center stand (still running), I tried to dial the idle back down, but:
- The idle screws felt extremely touchy
- Small adjustments made big changes
- I couldn’t get it to settle consistently
- I’m not confident I properly “reset” the idle screws from a true baseline
At this point, I’m not sure I’m correctly identifying when the idle screw is actually making contact internally vs just free play, so I may be chasing my tail there.
Current issue:
Right now, unless I keep the idle set relatively high, the bike wants to stall when I try to bring it down near ~1,000 RPM.
Additional observation (possibly related):
Yesterday, after running the bike and shutting it down, I put it on the center stand and realized I had left both petcocks on.
About 10 minutes later:
- Left side looked normal
- On the right side, I could see what looked like continuous fuel bubbling in the small glass area above the carb
I shut the petcocks off immediately.
Appreciate any guidance. I’m trying to be methodical here and not make things worse by guessing.
Keeping it real, if there were a spectrum of folks who are experienced and knowledgeable airhead mechanics, if it's not obvious, I'm pretty low on that spectrum. I have the original service manual as well as a copy of both a Clymer's and Hayne's manual. AI is a great resource for finding what I'm looking for as well. I came here, because on my clutch repair, some kind fellas helped me dial right into what needed to happen.
Additional observation.....when I turn the throttle it does not snap back....it usually makes it's way back eventually, but not immediately and consistently. Sometimes I have to turn it back myself
Idle setting adjustments should be performed when the motor is warmed up to full operating temperature. Valve clearance setting should be done when the motor is cold. All 70's airheads will eventually suffer from valve seat recession caused by the absence of tetra ethyl lead in the gasoline. This allows the valve clearance to tighten up as the valve head sinks into the valve seat. When this happens, the motor will not idle when warmed up.
I'm not familiar with D'ellortos. But, with Bings there are 2 screws that can effect idle, 1) the throttle stop screw and 2) the idle air mix screw. You want to be adjusting the throttle stops to set idle speed. The D'ellortos should have both of these as well.
If it were my project, the first thing I would do is set the valve clearances to spec and then go for a warm up ride and then fiddle with the finer adjustments.
"small glass area"? Is this an in-line fuel filter? If so, this would not be a concern. It takes several seconds for the fuel lines to back-fill when the motor stops running. If your fuel line plumbing has its right/left crossover line in place, you would not need both petcocks open to ride the motorcycle. You should ALWAYS shut the petcock(s) when not operating the moto.
Your throttle snap back could be improved with new throttle cables and by applying fresh grease to the hand set mechanism. I would recommend that you get accustomed to using you right hand to close the twist grip. It will never snap back crisply.
With regard to the factory service manual, it is written on the assumption that the user has been to BMW service school as a pre-requisite.
former Airmarshal, IL.
I have a /7, and there is a little “cruise control” thumb screw on the underside of the throttle control that puts drag on the throttle when screwed in. Don’t know if 90s has this - but that might be hampering your throttle return.
when the throttle is fully returned, there should be a bit of free slack in each cable, and the carb mechanism (not familiar with your carbs) should be resting against the adjustable idle stops. Idle speed is adjusted with the idle stops, completely independent of the cables.
This is also where the idle mixture screws are adjusted, with the throttles closed against the adjustable stops. On the Bings it’s kinda by ear, seeking the smoothest, happiest, highest rpm with each mixture screw. Once that’s done, you will have reset the idle speed again at the adjustable stops.
Finally, then comes the cables. Once off idle, being pulled by the cables, carb synchronization is adjusted by the cable length adjusters.
It’s a logical process.
All the above is VERY good info. In addition to that may I add these often overlooked service details....
If you have not owned the bike since new, then you'll need to go all the way back to the beginning. Your issue sounds to me like either too much home service, or not enough !
* Completely remove the twist grip. Is there white, splotchy corrosion on the chrome handle bar ? That white stuff is galvanic corrosion taking place between the diecast grip tube and the chrome when water (the electrolyte) gets in there. You must exclude the water and electrically separate these 2 metals. Slather a moderate viscosity grease all over (100% coverage) the bar end and inside the tube BEFORE reinstalling the grip. Red or blue chassis grease will do fine. White Lithium drastically changes viscosity over time and is NOT recommended. Put more of the same grease all over the 2 gears and the chain that pull the 2 cables. Now the twist grip should work as designed.
* Are there any "cable ties" securing ANY of your cables at ANY position ? These are usually installed WAY TOO TIGHT. Cut them off and start again. It's good to keep cables in their place, but cables move during operation. If you bind the cables in place (even though the intentions are good) the results will be bad... and a lot like you are experiencing. You can cinch down a wiring harness, but you cannot tightly constrain control cables. For cable ties that hold control cables, there should be enough slack in the loop to also insert your finger tip up to the first knuckle. You can only GUIDE the control cables; you cannot constrain the control cable(s).
* Lastly, when was the last time you lubricated your control cables ? I know, I know... the manual says they are Teflon lined and lubed for life. Yes, and I can sell you a big bridge in Brooklyn, NY ! Try feeding common ATF (automatic transmission fluid) from the highest point on the cable. Most ATFs are synthetic, thin, and do really well on control cables. Obviously getting the lube all the way down the cable can't be done in one session. Cable lubrication needs to be a CONSTANT focus every time the bike gets any service. If you will start adding 2-3 drops into each cable, then troublesome operation will soon be a thing of the past.
The owner must know and do these things. Getting a "full service" at any shop will not include these details.
I'm convinced that working the small details during service (of any type) are the times you'll notice the developing "big" problems. Conversely, working the "big items" doesn't usually do anything for the "small details".
Hope this helps.
Owning an old Airhead is easy.
Keeping an old Airhead running great is the true test.
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