The main metering system meters fuel from off-idle to wide open throttle operation. Under power operation it is supplemented by the power system. Its purpose is to provide efficient fuel metering during the cruising range of the automobile. Its operation is depended upon air flow through the carburetor venturi which, in turn, creates a low pressure in the venturi area, causing fuel to flow in the following manner.

As the throttle valves are opened further, allowing more air to enter the manifold and engine speed to increase, the vacuum decreases below the throttle valves so the air-fuel mixture below the throttle valves from from the off-idle holes gradually diminishes.

With the increased throttle opening, there is increase air velocity in the venturi system. This causes a drop in pressure in the main venturi which is increased many times in the boost venturi. Since the low pressure (high volume) is now in the boost venturi, fuel will flow in the following manner.

The low pressure in the boost venturi is transmitted to the tip of the main well tube or main discharge nozzle. Atmospheric pressure, which is greater, forces fuel from the float bowl through the main metering jets and into the main well. As the fuel passes through the main well tubes, it is mixed with air from the calibrated main well air bleeds. The fuel mixture then passes from the tip of the discharge nozzle, through the mixture passage, to the boost venturi and on into the intake manifold.

As the throttle valve opening is increased and more fuel is drawn through the main well tubes, the fuel in the main well drops. The calibrated holes in the main well tubes are proportionaly exposed to the air in the upper well area. When this occurs, they become air bleeds mixing progressively more air with the fuel passing through the main well tubes. Therefore, although the nozzle suction is increased at higher engine speeds, the air/fuel mixture to the engine remains constant throughout the part throttle range.

In some 2 Jet carburetor applications, the main well inserts are used. They are larger than the main well tubes and set in a recess beneath the venturi cluster. The main well inserts surround the main well tubes and have calibrated holes which are similar to those in the main well tubes. The purpose of the main well inserts is to break up vapor bubbles which may form in the main well during hot engine operation. This prevents the vapor bubbles in the fuel, caused by heat, from disrupting carburetor metering nd provides even fuel flow from the main well tubes and discharge nozzles.

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The main well inserts are used only on applications where engine heat causes excessive vapor bubbles in the main well area.

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Lower idle air bleeds are used in some 2 Jet carburetor applications to act as supplementary fuel feeds after off-idle, and during the main metering and power system operation. As the throttle valves are opened further past the off-idle discharge holes, air velocity past the lower idle air bleeds creates a vacuum and they begin to feed fuel. They continue to supply fuel during the main metering and power system operation, where they supplement fuel flow from the main discharge nozzles.

The purpose of the lower idle air bleeds is to supply additional fuel, to mix with the air, during the period whne the off-idle discharge holes cannot meet the fuel demands and the main discharge nozzles have no begun to feed sufficient fuel.

The lower idle air bleeds draw fuel through the idle system. Engine demands determine whether or no these supplementary fuel feeds are used in a particular carburetor model.