[1QuickS]

Good chance the mixture was overly rich and closing the mixture screw leaned the mixture to be closer to correct which will then cause the engine to run faster.

Idle jets may have been enlarged without erasing original size designation or the idle jet was not completely seated, either will create excessive fuel flow to mixture screw. It is possible that if you have a pre-1969 Weber body and the idle jet holder is designed for use with O-ring then this would cause idle jet to not properly seat.