Classic BMWs

[The Reverend Bluejeans]

Said speed screw.

[SiC]

I'm champing at the bit to get this back and start work on it. Not helped by the fact I have a whole load of Dolomite body work to finish first.

[hennabm]

What's the auxiliary air device like for reliability? Looks like it's something that can cause a whole manor of problems when it ages and stays closed because it's gone sticky from muck/carbon build-up. I'm surprised Bosch didn't go for an electronically controlled solenoid rather than relying on a heating element/engine heat to determine position.

Presumably it's a more analogue/mechanical version of an idle control valve solenoid that most later cabled Throttle Bodies got?

It relies on the heat and the vacuum. The one on my E30 is still the original.
They can give bother -depending on what AF/water mixture has been used over the decades. They are available - but not from BMW. I did see one recently on egay for around £180. Can't say how good it would be.
Best would be to take off the existing one and test it. Try to clean it up.

BTW the idle screw should not need adjustment - unless some bodger in the past has moved it to compensate for some other issue.

[The Reverend Bluejeans]

No vacuum involved. It’s water heated. It’s fully open when cold and closes up to reduce idle speed when warm by restricting the idle air circuit.