If the throttle positioner screw is turned in too far; the idle mixture screw will make very little change from all the way in to removed and in your pocket.
Two and one half turns is wide open. Anything past two and one half turns should effect no change.
Suggest:
Start engine and warm to normal temperature
Turn idle mixture screw in until thumb-tight
Back out 1 turn
Adjust idle speed with throttle positioner screw
There are other possibilities.
Jon.
