The main metering system utilizes the venturi principle to supply fuel through the primary bore from off-idle to wide open throttle operation.
As the primary throttle valve is opened beyond the off-idle range allowing more air to enter the engine intake manifold, air velocity increases in the carburetor venturi. This causes a drop in pressure in the main venturi which increases many times in the boost venturis. Since the lowest pressure (highest vacuum) is now in the smallest boost venturi, fuel flows from the main discharge nozzle.
At the boost venturi, the mixture combines with the air entering through the carburetor bore and passes as a combustible mixture through the intake manifold and on into the engine cylinders.
The fuel flow through the main metering system is metered between the tapered and stepped primary metering rod and the main metering jet. At light engine loads, the metering rod is held down in the main jet by engine manifold vacuum acting on the power piston and power piston spring. As engine load increases and engine manifold vacuum drops, spring pressure acting on the power piston overcomes the vacuum pull and gradually lifts the metering rods partially out of the main metering jets. This enriches the fuel mixture enough to give the desired power required to overcome the added load.
A calibration hole, called a 'top main well air bleed' is located in a casting projection in the air horn that covers the main fuel well. The hole bleeds air to the main discharge nozzle passage to achieve the desired air/fuel mixtures from the main metering system.
An adjustable part throttle screw is used in the float bowl to aid in the refinement of fuel mixtures for good emission control. This screw is pre-set at the factory and a plug is installed to prevent tampering and to seal against any fuel leaks. The plug should not be removed nor the screw adjusted in service.
The main metering system on 6 cylinder 2SE models operates the same as the 4 cylinder 2SE models, described above, except a side main well air bleed tube, pressed alongside the primary boost venturi, is used to bleed additional air into the main nozzle discharge passage for good nozzle response during main metering system operation. The side main well bleed is used in addition to the top main well bleed.