MFI Idle
 

On my 1970 911E 2.2 MFI-

I've had an ongoing issue with the the idle. After warm up, it settles to an idle around 850 rpm as long as the throttle is not touched. After a few revs, it idles at 1900 rpm and stays there. However, if I turn it off and restart the engine without touching the throttle, it's back down to 850 again. Very repeatable.

It also very occasionally runs up to 1900 and then drops to a normal idle, this may cycle several times. I can sometimes hear a click during the cycling. This only occurs when hot. There are no air leaks.

The thermostat washers are correct and clean so the warm up circuit appears to be working. The microswitch tests good also. There is no binding on the linkages either.

Any ideas what to look at next?

Sounds like your distributor is starting to advance around idle. It is common as the springs weaken over time. you may be able verify with a timing light.