[Gaz64]

The idle mixture screw adjusts the volume of air/fuel emulsion discharged through the idle mixture port as set by the idle fuel jet/air bleed combo, and works in the same principle on any fixed choke carburettor, more critical on IR manifolds with one venturi per cylinder.

The mixture screw still contributes while going into progression and does have a bearing on progression mixture, but as the throttle opens further it's contribution is proportionally less, depends on number of progression holes, certainly critical for the "popping on decel" syndrome, so as others have said in other threads 3/4 turn out is the norm.

Any more than say 1 1/4 turns out says the fuel jet is lean.

This general rule applies to any performance Weber carb, IDA, IDF, DCOE etc.