The model G two bore carburetor has an idle system to supply the correct air/fuel mixture ratios to the engine during idle and low speed operation. The idle system is necessary during this period because air flow through the carburetor venturi is not great enough to cause fuel to flow from the main discharge nozzles. Each bore of the carburetor has a separate idle system. They consist of idle tubes, idle passages, idle air bleeds, and the idle mixture adjustment needles or screws, and discharge holes.

In the conventional idle system, at idle speeds the throttle valve is cracked slightly open, allowing a small amount of air to pass between the throttle valve and bore of the carburetor. Since the engine requires very little air for idle and low speeds, fuel is added to the air by the application of vacuum (low pressure) from the intake manifold, directly through the idle system to the fuel in the carburetor floa tbowl. With the idle mixture needle holes located in a high vacuum (low pressure) area below the throttle valves and the fuel in the float bowl vented to atmosphere, the idle system oeprates in the following manner:

Fuel from the float bowl flows through the main metering jets into the main fuel well. It is then picked up and metered by the calibrated orifice at the tip of the idle tubes. It then passes up the idle tubes and is mixed with air from air bleeds located at the top of the idle tubes and in the idle cross channels in the venturi cluster casting. The mixture then passes downward in the idle channels through a calibrated restriction to the off idle discharge holes located just above the throttle valves. Here the fuel mixture is again bled with air and then moved to the idle needle holes where it is discharged and blends with the air passing the slightly open throttle valves and enteres the engine manifold as a combustible mixture. The idle mixture needle controls the amount of fuel mixture which enters each carburetor bore. Turning the mixture screw clockwise (inward) decreases the fuel discharge (gives a leaner mixture) and turning the screw counter-clockwise (outward) increases (enrichens) the fuel mixture. The number, size and location of air bleeds in the idle passages in the cluster castings, are determined by the engine idle requirements. These will vary between carburetor models.

On some applications, lower idle air bleeds are used in the idle system to supplement fuel flow after off-idle operation and during the main metering system operation. During idle they act as air bleeds. Operation of the lower idle air bleeds is explained under the main metering system.