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.