Abstract: A closed loop air-fuel ratio controller adjusts the air and fuel ratio of the mixture supplied by a carburetor to an internal combustion engine to a predetermined ratio in response to sensed exhaust gas oxidizing-reducing conditions. An air valve is energized to bleed air into the air-fuel mixture upstream of the exhaust gas sensor and independent of the mixture supplied by the carburetor when the carburetor idle fuel system is functioning and the magnitude of the adjustment in the mixture leaning direction reaches a predetermined value in response to a sensed rich air-fuel ratio to effect a decrease in the amount of the adjustment in the lean direction. The air bleed effectively increases the lean adjustment authority of the air-fuel ratio controller and avoids a lean air-fuel ratio excursion as the carburetor operation shifts from its idle fuel circuit to its main fuel metering circuit during cold engine operation.