Abstract: In an internal combustion engine having a carburetor, a mixture ratio control system by means of which the fuel-to-air ratio of the combustible mixture to be produced in the carburetor is regulated toward a predetermined target value and furthermore the pressure in the fuel delivery circuit of the carburetor is temporarily increased or decreased at an incipient stage during a period of time for which the fuel-to-air ratio of the mixture as detected from the exhaust gases resulting from the mixture is reduced below or increased beyond the predetermined target value.

Abstract: A temperature sensor is disposed at the upstream or middle portion of a reactor for oxidizing the unburned constituents contained in the exhaust gases discharged from the combustion chambers of the engine. The air-fuel ratio of the air-fuel mixture supplied to the combustion chambers is controlled so as to maintain the temperature of the upstream or middle portion of the reactor at the predetermined level in the vicinity of but higher than the temperature corresponding to the critical point where the temperature of the downstream portion of the reactor begins to decrease.