You have an idle adjustment screw in front of the fuel mixture assembly. Turn clockwise to reduce air flow and counter clockwise to increase.
If this does not have an effect AND assuming this only happens with the engine warm, then you may need to replace the auxillary air slide/valve. I just did this on my '79 450SL. With the engine coolant temperature reaches a certain temperature, the air slide does NOT introduce additional air into the mixture. If the valve is not closing then additional air is introduced THUS the faster idle speed.
There's also a balancing act between the fuel adjustment and the idle adjustment. The effect of having too much air when warm can vary depending on whether you're running too rich OR too lean...
btw: I learned all this from SteveFl's postings on the CIS fuel inject systems, etc...
1995 E300D 153K
1985 300D 142K (sold)
1979 450SL 122 miles (sold)
1992 500E 127K (sold)
1987 300SDL 132K (sold)
1986 300E 161K (sold)