During engine idle operation, air flow through the carburetor venture is very low and is not great enough to meter fuel properly from the main discharge nozzle.
Therefore, the idle system is used to provide the proper mixture ratios required during engine idling and low speed operation.
Carburetor Idle System of Classic OLD Cars
The idle system consists of an idle tube, idle passages, idle air bleeds, off-idle discharge ports, idle mixture adjusting needle and the idle mixture needle discharge port.
Don’t get nervous. In the idle speed position, the throttle valve is slightly open, allowing a small amount of air to pass between the wall of the carburetor air horn and the edge of the throttle valve.
Since there is not enough air flow for venturi action, the fuel is made to flow by the application of vacuum directly through the idle system to the fuel in the carburetor bowl.
This is accomplished by the low pressure below the hrottle valve (manifold vacuum) causing the fuel to flow through the idle tube from the float bowl into the idle passage, where it is mixed with air from the air bleed.
You recall what is meant by air bleed?
What is the cause of the low pressure below the throttle valve? It is the manifold vacuum.
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This is the low pressure caused by the suction of the pistons on their intake strokes. When the pistons move down, they create a partial vacuum which the outside air tries to fill. But the almost closed throttle plate restricts the air flowing past and causes an expansion of the air filling the space created by the piston, producing pressure in the manifold which is less than atmospheric. A partial vacuum is formed, the manifold vacuum.
The air bleed just mentioned is the first stage of atomizing the fuel. The mixture continues down the idle passage, past the off-idle ports.At this point, these ports act as air bleeds to further break up the mixture because the throttle plate is positioned such that the off-idle ports are still on the atmospheric side.
The mixture flows past the mixture screw into the carburetor bore and into the engine intake manifold.With one end of the idle passage open to this vacuum, the other open to the fuel in the float bowl, and the air bleeds along the way, the difference in air pressure will force fuel into the air stream through the carburetor.
The mixture screw controls the idle mixture ratio.As the throttle valve is opened and allows more air to flow during low speed or off-idle operation, the off-idle ports are exposed to manifold vacuum.
At this time, they begin feeding extra fuel for off-idle (slightly higher than idling) requirements. Thus, the off-idle ports have a dual purpose. At idle, they act as air bleeds but during the off-idle range they change to fuel mixture feeds.
Article Source : This article courtesy should goes to Auto Mechanics Autodology – Technical instruction manual by System Operation Support.