I have seen manual adjustments on early "Grey market" 500 motors but never an 85. But, if you have an adjustment you must have the old manual system with thermostatically controlled additional air valve.
Engine speed is determined by the amount of air going in. Fuel control and ignition can reduce speed by reducing performance only. If your car runs at 1500 rpm warm it should run faster cold because the additional air valve would be open. It seems your valve may be open all the time. This makes me think your mixture is so rich cold that performance drops and the speed isn't high.
By reducing the idle air, hot, you are are in effect futher enrichening the cold mixture. This is all I gather from your info.
Your starting point should be the high warm idle, where is the air coming from to run that fast? Verify your mixture, the methods of which will vary depending on whether your car was modified for US emissions.
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician