At idle speeds, the throttle valve is nearly closed so there is not enough air flow through the venturi to lift fuel from the float bowl. To supply enough fuel for idle and off idle requirements, an idle system is used. To make fuel flow, manifold vacuum is applied directly to the fuel in the bowl from the idle needle hole and the off idle port as the throttle valve is gradually opened. The idle system consists of the idle pick up tube, idle tube, idle passage, air bleeds, idle mixture needle, and idle discharge hole.
Atmospheric pressure acting on the fuel in the float bowl, forces fuel through the main metering jet into the main well. The fuel then travels up through the idle pick up tube and through the cross bar in the air horn. Air is then bled into the idle fuel at the center of the cross bar through the two top bleeds and the main nozzle. The air/fuel mixture then is picked up by the horizontal idle tube in the cross bar and metered through a calibrated restriction, then passes on into the vertical down channel where it is further bled with air by an internal air bleed in the verticle channel in the top of the float chamber. The air/fuel mixture continues downward past the off idle discharge port there more air is picked up to mix with the fuel mixture and then passes out the idle needle hole below the throttle valve. Here, the fuel mixture mixes with air coming past the slightly open throttle valve and passes into the engine as a combustible idle mixture.
On some applications an anti-bubble screen is used inside of the main fuel well. The purpose of the antibubble screen is to break up any vapor bubbles which might form in the main well during hot engine idle and disrupt proper fuel metering.
As the throttle valve is opened slightly and engine speed increases, extra fuel is needed to combine with the additional air going by the throttle valve into the engine. This fuel is supplied by the off-idle discharge port. This supplies the fuel to the engine until air velocity is high enough in the venturi area to obtain efficient metering from the main metering system. Further opening of the throttle valve causes increased air flow through the carburetor bore, which causes pressure drop in the boost venturi sufficient to cause fuel delivery from the main nozzle. It should be remembered, however, that idle port discharge does not cease at this transfer point, but rather diminishes as the main nozzle discharge increases. Thus, the two systems interact and produce a smooth fuel flow at all engine speeds.
Using a vacuum gauge.
By ear.