The primary idle fuel system, functions when the engine is operating at low engine rpm. It supplies the air-fuel mixture when the air flow past the carburetor venturi is sufficient to operate the main metering system. At curb idle speeds, the throttle plates are almost completely closed and, with manifold vacuum below the plates, enough difference in pressure is created between the fuel bowl and the idle discharge port is a vehicle slot-type port which extends slightly below the closed throttle plate. When opening the throttle plate, a greater portion of the upper discharge port is exposed to manifold vacuum, thus, a larger amount of idle air-fuel mixture will discharge from the upper port. Further opening of the throttle plate results in a decrease in manifold vacuum and a decrease in the quantity of idle air-fuel mixture that is discharged. As the idle system tapers off, the main fuel metering system begins to discharge fuel.