300 SD idle screw

On my '79 300 SD my idle is too fast. My linkage and fast idle knob on the dashboard are all adjusted ok. I tried to lower the speed by unscrewing the idle screw on the side of the IP and it had no effect on slowing the idle. Turning it in increases the idle normally. I can remove the screw, and the idle remains the same. I have looked at the tips of both the idle adjustment screw(it's tip was flat), and the rack dampener screw(it's tip was slightly concave), I checked the idle adjustment screw while unscrewing the rack dampener screw. I noticed no ability to lower the idle, I noticed no change in idle upon unscrewing the rack dampener either. What other things control idle speed?

[Edited by patsy on 12-04-2000 at 02:40 PM]