R26 variable idling

[BSA Bob]

My R26 is now running, but the idling is poor. It varies in speed from OK to quite fast even with a closed throttle. If the throttle's opened & then shut the speed takes a long time to slow, sometimes it doesn't slow at all. It seems to get worse as the engine heats up. The carburettor's a Bing 1/26/93 with an accelerator pump.
The idling controls on the carburettor are set OK & the engine's been static timed.
Can someone help with ideas?

[BSA Bob]

[Elefantentreiber]

Sounds like a worn throttle slide causing too much bypass air past the closed slide

[rolf.soler]

check out this well known problem:
https://bmw-einzylinder.de/forum/index.php?topic=8369.msg109397#msg109397

[Elefantentreiber]

may be or not, this is the first time, in my over 40 years involved with BMW, I saw this problem.

But why not.

check out this well known problem:
https://bmw-einzylinder.de/forum/index.php?topic=8369.msg109397#msg109397

[Kurt in S.A.]

Does the slide freely move up and down in the bore?  With the engine not running, does the slide return to the bottom when the throttle is released?  I recently replaced the return spring inside the carb of my R25/2.  It appears that the original one was sacked.

Kurt

[rolf.soler]

may be or not, this is the first time, in my over 40 years involved with BMW, I saw this problem.

maybe you work to little with one-cylinders ?
There's is quite a number of people who had to fix the "hochgewandernter Mischkammereinsatz"
and it causes precisely the problem of difficult idle setting

[BSA Bob]

Thanks for these replies.
The slide doesn't have any noticeable play & returns to the bottom of its travel. Fortunately there's no sign of the problem mentioned by Rolf. I thought it may have been a vacuum leak but spraying WD40 & gas over the jointed areas did nothing.
But something seems to've gone right. I stripped & cleaned the carb again using cleaner & compressed air, & also added a bit of oil to the centrifugal advance. So far this has fixed things with the revs dropping properly & a consistent idle.