How A Carburetor Works: Idle Circuit

When the throttle valve is closed to slow the engine to idle speed, the main discharge nozzle stops supply fuel, and the engine will stall. The nozzle fails to feed gasoline because, when the throttle is closed, the rush of air through the air horn is cut off.

An additional passage branches off the main discharge tube and runs up through the low speed jet, across, down through the economizer jet and on to the idle port. From the idle port it runs down to the idle screw port, where it enters the air horn slightly beneath the throttle valve.
Since there is no vacuum above the throttle but a strong vacuum below, the vacuum will pull at the idle screw port.
The vacuum will draw fuel from the bowl up through the low speed jet, which calibrates or controls the amount that passes. As the fuel is being pulled toward the economizer, the vacuum is also drawing air in at the bypass and air bleed openings. The fuel is thoroughly mixed with this air and travels down through the economizer, which further meters the flow and tends to mix it better. The fuel and air proportions are controlled by the size of the air bleed, bypass and low speed jet openings.
Moving down through the passage and idle port, the mixture arrives at the idle screw port. The amount that can be drawn into the engine is determined by the position of the idle adjustment screw. When screwed in, it reduces the amount and when backed out, it admits larger quantities. To adjust, the idle screw is turned in until the engine misses and then turned out until the engine ‘rolls.’ The screw is then set halfway between these two positions. The engine should now idle smoothly.

Some idle mixture screws are covered by tamper proof covers and set at the factory. Should you need to remove the cover, there is no way to replace them.

Updated on 04/27/2022

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