Weird problem
 

Just got back from my Nordkapp trip (report and pictures to follow later, stay tuned!), and during that trip all of a sudden it started to run weird at idle.

- It is slow going down from ~2krpm to idle.
- Once at idle, it is all over the place between 1200rpm and 500rpm, going lower until it stalls
- It shakes and sounds like it is sometimes firing on 1 cylinder, which would explain it going up and down in rpms
- When cold, it idles normally.
- Off idle, it runs great as ever.

I am suspecting a dodgy lower spark cap, but that does not necessarely explain all symptoms. I haven't had time to look at it while I am back, nor to replace the spark cap. But it would help if the collective could give me a few more hints to check, or to confirm the cap is indeed the most likely suspect.

Somebody snuck an R1200S motor in the frame when you weren't looking.

geo

Air leak... possibly related to the TB vacuum port. Add a TB-to-TB crossover between those ports.

I'm with you on the spark plug. I have a simple suggestion. With the bike idling, gently push on the throttle cable on each cylinder, independently. I do this where the cable is exposed where you make the adjustment to balance the TB's. If one cylinder responds 'normal' and the other stumbles, you've narrowed it down to the stumbling cylinder. I used this technique 2 weeks ago to zero in on a bad coil.

If you can't get it to idle, will it idle with the fast idle set or with a throttle lock?

If it's stumbling that badly, I suspect a primary coil, not the secondary. When I diagnosed my bad coil 2 weeks ago, the primary coil was totally dead - no spark, period.

You can always try this:

http://forums.pelicanparts.com/bmw-r1100s-r1200s-tech-forum/422272-troubleshooting-dual-spark-ignition.html

Reason I figured the cap: when doing a checkup before the trip, I noticed the lower cap would not 'click' on the plug like it did on the other side. I squeezed the clamp a bit with pliers (sp), but that did not make any change.

Also, while idle is bad, normal operation and fast idle are ok. The bike hauls ass. If one of the primary coils was bad, it would not run properly at all.

I had a similar problem after a tank of bad gas. I drove 4000 miles to get home, with the fast idle on so it would not die. Pulled the TBs and cleaned them with carb cleaner. It was fine after that.

Good luck-

...as long as it isn't my motor... :D





http://i145.photobucket.com/albums/r...8Image0007.jpg

Top to bottom (but I don't know what's in a R11 - mind)

1. Idle actuator (was my ugly idle case - Munich accepted for 2nd time my claim/guesswork)

2. Primary coil (happens after cleaning/rain/water/typhoon)

3. TPS sensor (just happens).

4. unbalanced throttle cables (least possible)

Had a very similar problem on a long trip last year. Turned out to be a leak in the cap I put on the throttle body nipple when I "lost" my evap. canister.