At idle and low speed the manifold vacuum is at it’s highest which brings fuel in from the idle circuit. Do not confuse this with venturi vacuum. Venturi vacuum at idle isn’t strong enough to pull from the main system so additional fuel is necessary.
Both barrels have identical idle tubes and passages and work simultaneously.
Because of the reduced pressure in the intake, fuel will flow from the float bowl to the intake. Fuel from the float bowl flows into the main well which then into the idle well. It then moves upward through the idle tube in the idle well and passes into the idle passage. The restriction in the lower tip of the idle tube and the restriction in the top of the idle passage in the main body meter the flow of fuel in the idle system. Directly above the horizontal channel at the top of the idle passage in the main body is the idle air bleed. The air bleed adds air to the idle system. This is also the idle vent. Without this vent gas will be siphoned at high speeds or when the engine is turned off.
The air/fuel mixture travels down the idle passage to the idle discharge and idle transfer holes. The idle discharge hole is below the throttle plate which allows fuel into the intake at idle. As the throttle plate is opened, the idle transfer holes are uncovered allowing more fuel into the intake.
Idle Mixture Adjustment
Idle mixture adjustment is done on the car at operating temperature.
Turn each idle mixture screw in equally 1/4 turn at a time, until the RPM starts to drop, then turn the screws back out 1/4 turn.
Another method is to hook up a vacuum gauge to the intake and adjust the idle mixture screws to get the best vacuum and a steady vacuum needle.
If turning the mixture screws don’t change the RPM then you have either a vacuum leak, or maybe a clogged idle passage.