1968 gts avs carburetor

Exactly. The idle mix is only for fine adjustments to bring AFR to 14.0 or 14.2 using a CO meter. It says in the service manual.
You might see a slight effect on a vac gage or sensitive tach.
also
1968 Carburetor Service Highlights Service Book - Session 243
Filmstrip
1968 Carburetor Service Highlights

Engine Performance Diagnosis (Session 250) from the Master Technician's Service Conference
Filmstrip
1968 Engine Performance Diagnosis from The Master Technician Service Conference