Forum bmwbike.com

Lately my engine starts to idle up to 2000 rpm after prolonged use and engine warmed up after about 20-30 min regular driving.When starting out the idle is at approx.1200 rpm. The engine runs smooth
all along just keeps creeping up faster in idle.Maybe someone had a similar incident?
Thanks Al.

It is normal for RPMs to increase as the oil temperature increases and viscosity decreases, from cold idle to hot running.
As I remember from my /5-/6-/7 bikes which I no longer have, I used 20W-50 oil and engines would idle cold at about 750 RPM which would increase to just under 1000 RPM when hot.
If your cold idle is 1200 RPM, I can see how it might be 2000 RPM when hot. Adjust your idle stop screws to lower the RPMs.
(All my bikes are now /2s using 40W oil, and their RPMs also increase when hot.)

Thanks for your reply.I do own this bike for the last 30 years and I'm aware of the cold verses hot running. After start up I have to run it with the choke partially closed because for seconds the engine w0uld only idle when cold less then 700.Soon it smoothes out and warmed up it runs around 1000-1200 and it used to hold at this mark until lately.Like I said it will take a while till gradually it rises. I still have the original diaphrams in there,but I don't believe that they are bad,since the overall running and performance is not affected-or.

So this is a recent development. Does the RPM increase slowly to 2000 by itself, or does the RPM become "stuck" at 2000 after revving the engine?

If the RPM increases by itself to 2000 when hot, it may be that that RPM is what the carb settings are calling for. The adjustment of the carbs could have changed over time. I would adjust the carbs so that hot idle is 900 RPM. If you cannot adjust for 900 RPM hot, that might indicate a problem.

If the RPM becomes stuck at 2000, perhaps the centrifugal advance is not allowing the timing to return to the S mark. The advance could need some lube, or have weak springs, or be worn so that it traps the weights.

It is a gradual rise and can't be lowered once the engine has been running for a while.I'm sure it's heat related.I just had an idea ,since the carbs feel hot too.I know because of the location they will get hot,but I thought they feel too hot and can't be touched.MAYBE the connecting hose is too hard by now and transfers too much heat which in turn makes em act up.
I think I will ask someone at BMW next month while I'm in GER.

Hot carbs is not an issue that I have ever heard of. Are you saying your carbs become as hot as the valve covers, too hot for a long touch?
If you can't reduce your hot idle from 2000 down to 1000 RPM, I would think that there is some adjustment problem other than heat.

I just found the cure for my idle problem.I changed the springs for the advance weights and my baby is back to normal. The old springs showed no signs of any irregularies, but were just getting weak when hot from old age.