valve 3 is greater than the opening a when the engine is idling after it has been warmed up therefore air intake may be sufficiently increased before the engine is warmed up so that smooth engine performance may be ensured in a conventional choke opening mechanism when the vehicle is decelerated with the choke valve in its fully opened position the opening of the throttle valve will be at a position labelled a in fig2 when the vehicle is stopped the opening of the throttle valve is however too small for an engine which has not been sufficiently warmed up and unstable idling will occur however according to the present invention the choke valve is not fully opened when the vehicle is running with the engine not sufficiently warmed up that is the choke valve is opened only to such an extent where no choking effect is obtained thus a satisfactory counter measure overcoming increase of pollutants in the exhaust gases may be employed without modifying the conventional fast idling mechanism of the engine and without adversely affecting engine idling before complete engine warm up is achieved after the engine is sufficiently warmed up the choke valve will be moved to its fully opened position by operation of.