the heater 35 is not limited to said exhaust gas sensor for example it may be controlled by a signal which simulates the operating conditions of the engine further the heater 35 may be energized when the r p m of the engine reaches a predetermined value in the above embodiments the system used has been one for compensating the difference pressure adjusting spring for setting the pressure difference setting diaphragm but the compensation of air fuel ratio may also be achieved by varying the pressures p 1 and p 2 in the suction pipe acting on opposite sides of the pressure difference setting diaphragm 11 fig5 shows such embodiment wherein the upstream side of the upstream valve 5 and the intermediate chamber 8 downstream of said valve communicate with chambers a and d through a venturi 36 and a chamber b communicates with the venturi 36 at a position where its static pressure is detected designated at 37 is an on off valve which is opened when it is desired to increase the fuel proportion relative to the air when the engine is at full throttle or warming up when the engine is at full throttle the opening and closing of said valve 37 is effected by an electric contact set placed on the flow control valve 5 or a pressure sensitive switch placed in the manifold and adapted to be closed when the negative pressure in the manifold is high.