RPM fluctuating at idle

[Ccporsche]

The RPMs in my '79 930 are fluctuating between 800 and 1200 while idling. It constantly fluctuates, revving up to 1200 from 800 over and over.

Any idea what the problem could be?

[T77911S]

too rich

check for 12v at the warm up reg (WUR) would be a first check

[JFairman]

yup, too rich.
All mechanical air flow meter fuel injection systems - sensor plate like CIS has or flapper door controlled like L-jetronic has will do that when they are too rich.

[mark houghton]

As suggested, do check that you have power to the WUR, to make sure it's warming up and leaning out as it should. Also, if you have the gauge setup, you should check system and control fuel pressures (cold and hot) against specifications. This is particularly warranted if this problem suddenly appeared.
Short term, you can adjust your mixture slightly leaner at the mixture control point on the fuel head. 3mm allen wrench, push down until it engages, and turn counter clockwise just a degree at a time.

[lee88]

Related question... I realize "surging" idle RPM as the OP described is a symptom of rich fuel mixture. But how, if at all, would the presence of an A/C compressor impact how one sets idle and fuel mixture on a CIS car like our 930s? Obviously if the compressor is running, you have a small extra load on the engine at idle.

Lee