There is no adjustment for idle speed - it is controled by a small
analog computer (control unit) through an idle speed air valve.  You
can adjust the mixture by way of a (iirc) 3mm hex wrench in the top of
the fuel distributor.

These affect idle speed:
- Leaks in the intake side - manifold gaskets, vacuum hoses,
diaphrams, seals, etc.
- Coolant temperature sensor
- Bad wiring, especially bad grounds
- Sticking idle control valve
- Bad news - most common problem is the expensive control unit, but
there are people who rebuild these for reasonable cost - search this
group for sources.

Diagnose first - you will run out of money before they run out of
parts.  The shop manual will give you diagnostic steps.  Good luck.

Mike - '82 380 SL