That is, they utilize this higher pressure to atomize and force fuel out of the top hole to maintain an engine idle setting. As the throttle is opened and the air consumption of the engine increases, more fuel is needed but the differential pressure between the bowl reservoir and the top discharge hole decreases. However, at the same time this seemingly detrimental sequence unfolds the plate has moved past the second hole, placing it in a low pressure region and transforming this orifice into a needed fuel jet. This continues for the third hole, until the plate is opened sufficiently to allow the interaction of the air through the venturi to discharge sufficient fuel from the main nozzle. The idle screw on the side of the carburetor throttle body has a needle point that extends into the top orifice.
Adjustment opens and closes the circuit to calibrate the fuel discharge for proper engine idling.
The timing of the operation of the idle circuit is critical to the performance of the engine as the throttle is advanced. Without the holes there would be a pronounced lean condition that would manifest itself as a severe engine stumble if speed is increased too rapidly. There is some misconception about how fuel is discharged from a standard float carburetor. The proper understanding begins by noting that the outlet of the main discharge nozzle is located in the center of the venturi and that fuel in a carburetor only flows when subjected to a differential pressure. When the throttle is advanced, opening the plate in the carburetor, the amount of air through the venturi is increased.