The carburetor supplies the engine with the correct mixture to suit all operating conditions, from idling to high speed. To do this it has a number of systems.
With the engine stopped, the throttle valve is closed but fuel is still held in the bowl. With the engine running, fuel is held at a set level by the float and a needle valve. Fuel supplied to the engine when it is running is replaced by fuel from the tank.
A passage from the air horn to the float bowl balances air pressure between the air cleaner and the float bowl. The idle air passage and the main discharge nozzle are above the level of the fuel.
The first stage of the idle system uses the idle and low-speed circuit, plus an idle adjustment screw.
The throttle valve is almost closed, so air-flow through the carburetor is very small. The action of a piston creates a low-pressure area below the throttle, and this is concentrated at the edge of the throttle valve as the air passes the idle port. Fuel flows from the float bowl, through the idle passages and into the carburetor below the throttle valve at the idle port. The air bleed lets air enter the fuel on its way from the float bowl. This helps aerate the fuel before it reaches the idle port.
Engine idle speed is set by two different adjustments. The amount of fuel is adjusted by the mixture adjustment screw at the idle port.
The amount of air is adjusted by changing the throttle stop screw.
Second-stage idling starts as the throttle valve opens. This is similar to idle, but with the low-speed port uncovered. Both ports discharge fuel to mix with incoming air. Low-speed ports help the transition from idling, to low-speed, to high speed. Without them, the engine tends to hesitate until the main system comes fully into action.
