Idle speed too high

The idle speed on my 1980 450SL is too high at about 1400 RPM. The idle speed will decrease to about 1100 RPM after driving a while. The idle speed screw will not get it below this speed. The thottle linkages appear in order and do allow the throttle to close properly. Recent repairs included a new ignition control unit and coil and spark plugs.
What things can I check that may effect the idle speed? Timing reads 5 BTDC with all vacuum lines connected. This car is a project car that was not running when I got it due to electrical problems. Thanks for any advice you can offer.

Have 1975 450 SL that suffered the same problem.
I assume you have found the idle adjustment screw and it is already turned in all the way.
If so there are two elements that can effect idle speed that you need to check.
The experts assured me that my problme was caused by an electricly controlled fuel valve located at the idel control assembly. I changed it it was not the problem. If you change yours, let me advise you that there will be fuel under pressure up stream from it. So do it with a cold engine.
My problem turned out to be the thermostatically controlled slide valve also located in the idel cotrol block assembly. It was carboned up and jambed. This explains why your idel speed drops a bit when the engine is hot.

Good luck