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When the engine is idling, the throttle plates are nearly closed which creates a strong vacuum in the intake manifold below the throttle plate, the suction from this vacuum draws the fuel from the fuel well and into the throttle bore below the closed throttle plate. The fuel passes up a dedicated fuel gallery located between the emulsion tube well and the fuel gallery running down the outside of the throttle body. It then passes through the idle jet where atmospheric air from the idle air bleed jet mixes with and emulsifies it before continuing down the external fuel gallery. The resulting emulsified air/fuel mixture flows out of the metering hole controlled by the idle mixture screw and the progression holes in the inner throttle body wall below the edge of the throttle plate. The idle mixture screw is a flow controlling needle valve with a tapered tip that mates with a small hole in the throttle bore and once set is secured with pressure from the compression spring wrapped around it. When the throttle plates are closed during idle operation the first (lowest) progression circuit hole should be blocked by the edge of the throttle plate leaving only the fuel from the idle mixture screw port available for running.

As the throttle plates are opened up and the engine speed increases there are more holes of the progression circuit exposed to the vacuum below the edge of the throttle plate. The additional exposed holes supply more fuel to match the increased airflow from the opened throttles. However, the vacuum below the throttle plates decreases with opening of the throttles until eventually the vacuum is no longer sufficient to continue to draw fuel from the progression circuit. Eventually all of the progression holes are exposed to the air flow past the opened throttle plates and with larger throttle openings the fuel flow out of these progression holes essentially ends. Before ending fuel delivery via the progression circuit, the main circuit begins fuel delivery, this simultaneous region of fuel delivery operation is referred to as transition. Higher engine speeds are only possible by fuel delivery from the main circuit.

Remember that the idle and progression circuit emulsifies the fuel delivered to the engine just as the main circuit does with its air correction jet mixing air with raw fuel via the action of the emulsion tubes. The idle and progression circuit achieves the same result using a different method; air from the idle air bleed jet is mixed with the raw fuel inside the body of the idle jet. In addition to this initial emulsification of fuel there is additional emulsification that is far more subtle; progression holes above the closed throttle plate are essentially at atmospheric pressure and those that are below the edge of the throttle plate are exposed to the vacuum of the intake tract. Therefore, those holes above the throttle plate actually provide additional air to the emulsified fuel in the fuel gallery which leans out the mixture delivered to those holes below the throttle plate. As the throttle plate is opened and more progression holes are exposed to the vacuum, there are fewer holes exposed to atmospheric air pressure above the throttle plate, this decrease in the number of holes to atmospheric air pressure decreases the air added to the fuel in the fuel gallery. This is thereby an enriching action and corresponds to the need for a stronger fuel mixture strength with the increased airflow of larger throttle openings.