The main fuel metering system is calibrated to supply a leaner air-fuel mixture than the other fuel metering systems. This is because the main fuel metering system operates during a cruise or part-throttle condition and under low engine power-load requirements. Main system calibration is conrolled by calibrated main metering jets, high-speed air bleeds, and the fuel level in the fuel bowl. The system begins to function when the air flow through the carburetor venturi creates a sufficient pressure drop (vacuum) to start fuel flow in the main system. The pressure drop at the main discharge nozzle in the booster venturi will increase as the air flow through the carburetor increases. When engine speeds increase, the fuel flow in the main system will increase.