A device for adjusting the intake air temperature of carburetor-equipped internal combustion engines having an air intake system with a flapper valve in a duct junction admitting cold raw air and/or preheated raw air to the intake filter, depending on the raw air temperatures, the flapper valve position being controlled by a pneumatic membrane actuator receiving negative pressure from the intake manifold, via a vacuum line with a thermostat-controlled relief valve responsive to the temperature of the air mixture. In the drive connection between the membrane actuator and the flapper is arranged a wax thermostat which increases or decreases the effective length of the connection in response to the ambient air temperature, thereby adding the thermostat displacements to the actuator displacements.