Fuel enters the idle circuit through the main jet and moves through the idle tube. Fuel is pulled by vacuum, which is at it's highest at idle speed.
Make sure the idle air bleed, idle transfer hole and the idle discharge hole are clean and clear.
Adjust the idle mixture only at idle speed. Once the RPM gets over idle speed, the idle circuit is bypassed.
To adjust the idle mixture attach a vacuum meter to either the carburetor, or intake manifold and adjust for smooth idle at the highest vacuum reading.