The Venturi effect is the reduction in air pressure and therefore the increase in air velocity when filtered air flows through the throat section of a simple carburetor. The reduction in air pressure and therefore the increase in air velocity in the throat section of a carburetor is limited by the choked flow condition at Mach number unity. A fuel nozzle is placed in an appropriate location in the throat section to atomize the incoming fuel and mix it with air. A float in a stored fuel chamber adjusts the fuel level with respect to the throat location. Upstream of the throat area, a choke valve controls the incoming air mass flow rate to regulate a lean or rich air/fuel mixture, A/F = ṁair/ṁfuel, for load requirements and environmental conditions. Downstream of the throat area, a throttle valve opens and closes as actuated by the accelerator pedal, to supply the required A/F mixture to the engine inlet manifold. The ideal A/F mixture for a simple carburetor is 14.7, namely when the air flow rate-to-fuel flow rate ratio is 14.7 and the incoming fuel and oxygen will be consumed completely during the combustion process. When there is excess air flow through the simple carburetor (i.e. A/F > 18), the mixture is too lean for combustion. When there is excess fuel flow through the simple carburetor (i.e. A/F < 10), the mixture is too rich for combustion.
