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