A device for automatically regulating the intake air temperature of a carburetor-equipped internal combustion engine whose air intake system includes a flow proportioning pivotable flapper in a duct junction admitting cold raw air and/or preheated raw air to the air intake filter, depending on the ambient temperature, the flapper position being controlled by a pneumatic membrane actuator receiving negative pressure from the engine intake manifold, via a vacuum line with a thermostat-controlled pressure relief valve which reads the temperature of the raw air mixture. In the cold air intake duct is arranged a temperature-responsive unit, in the form of either a wax thermostat or a bimetallic spiral and disc cam, which serves as a position-adjustable stop thereby preventing the flapper from completely closing the warm air intake duct, when the engine takes in cold air, operating at full load.