As engine speed increases, the air passing through the booster venturi creates a vacuum. The amount of vacuum is determined by the air flow through the venturi, which in turn, is regulated by the speed of the engine. The difference in pressure between the main discharge port and the fuel bowl, causes fuel to flow through the main fuel circuit. At a predetermined venturi vacuum, fuel flows from the fuel bowl, through the main jets, and into the bottom of the main well. The fuel moves up the main well tube past air bleed holes. Filtered air from the high speed air bleed enters the fuel flow in the main well tube through holes in the side of the tube. The high speed air bleed meters an increasing amount of air to the fuel as venturi increases, maintaining the required air-fuel ratio. The mixture of air and fuel is lighter than raw fuel and responds faster to changes in venturi vacuum. IT also atomizes more readily than raw fuel. The air and fuel continue up the main well tube past another air bleed which also acts as a vent to prevent siphoning when the engien is shut down. The fuel is discharged into the booster venturi where it is atomized and mixed with the air fuel mixture admitted to the intake manifold, regulating the speed and power output of the engine. A balance tube is located in each venturi directly below the booster venturi. When decelerating, the balance tube siphons off any excess fuel droplets remaning around the edge of the booster venturi and discharges the droplets into the equalizing slots in the base of the carburetor where they are mixed with the idle fuel. THe balance tube also acts as an additional air bleed during the idle fuel system operation.