The black plastic knob sets base idle not actual running idle. Running idle is controlled by the computer through the air control valve at 750RPM with AC off and 900RPM with AC on. Base idle is set at 700 with this black knob which bypasses more or less air around the throttle body butterfly. Base idle is set by disabling computer idle speed control by grounding red and white test wire at passenger side suspension tower. If I remember clockwise decreases speed.

There is also a mechanical throttle stop screw on the throttle body which should be set to stop the linkage movement just as the butterfly reaches fully closed position thus not putting any strain on the butterfly and it's shaft. Some mechanics?? mistakingly use this screw to set idle speed.

Hope this helps.