The idle system controls the flow of fuel at idle speed and at slow speeds until the throttle is opened wide enough to allow the power system to operate. When the throttle plate (6) is in the idle position, it is slightly open with the idle of the plate between the primary and secondary idle orifices (7) and (8). With the plate in this position, the air pressure at the primary orifice is lower than the air pressure in the fuel bowl chamber, due to manifold vacuum at the primary orifice. Therefore, fuel is forced from the fuel bowl into the idle passage (10). As the fuel travels through the idle fuel passage, it passes through the idle jet (11) to the point where it is combined with air entering the idle fuel adjustment needle seat (12). The fuel is mixed with air again when it passes the secondary idle orifice as it travels through the idle fuel passage. The mixture that emerges from the primary idle orifice is rich. However, it is reduced to the correct proportion by the air passing the throttle plate.
As the throttle plate is slowly opened from the idle position, the secondary idle orifice is gradually subjected to greater manifold vacuum, and as the manifold vacuum increases, the orifice ceases to bleed air to the idle fuel passage. Instead fuel is fed from the idle fuel passage through the secondary idle orifice tot he engine. With the throttle open wider than idle, a greater amount of air is admitted, requiring a greater amount of fuel than can be supplied through the primary idle orifice. The additional fuel supplied through the secondary idle orifice maintains the correct fuel air ratio for the engine.
As the throttle is opened wider, the idle fuel delivery begins to fade and the power fuel system starts to function.
When the throttle plate is closed during idle, air flow through the venturi is minimal - too little to draw fuel through the main discharge nozzle. The Marvel Schebler carburetor solves this by routing fuel through a dedicated idle circuit.
Fuel is drawn from the bowl, metered through the idle jet, mixed with air from the idle bleed, and then flows through internal passages. At idle, fuel exits through the idle discharge hole and the transition port, ensuring a smooth and steady idle.