Turning the idle mixture screw reduces the amount of fuel.
Inspect the idle mixture screw for any grooves. Replace if any are present.
To adjust the idle mixture:
Before installing your carburetor back on the car, run in the idle mixture screw until it gently seats, then turn back out 1 1/2 turns.
Bring the vehicle up to operating temperature.
Set the idle to specifications.
Turn the idle screw in 1/4 turn and let the engine catch up.
When the RPM starts to drop, turn the screw back out 1/4 turn.
It’s OK to adjust the screw slightly to get a smooth idle.
Number one cause of a rough idle is one or more weak cylinders, or a bad electrical part such as the spark plug.