AIR-FUEL RATIO CONTROL DEVICE

PURPOSE:To accurately control the air-fuel ratio in a region, in which the engine speed is low and the openings of throttle valves are large, by a method wherein the cross sectional area of an air suction side passage led to Venturi in a carburetor equipped with a single barrel and fixed Venturi. CONSTITUTION:The air, the flow rate of which is almost determined by the rotational speed of an engine 1 and the respective openings of throttle valves 6-9, are sucked in the respective cylinders with the rotation of the engine 1. Furthermore, the negative pressure, which is determined by the suction air flow rate, is developed at a Venturi part 10 and led through a negative pressure lead- in hole 25 and a conduit 24 in the negative pressure chamber 23 of an actuator 20. In this case, when the negative pressure becomes excessive with the increase of the air-flow rate passing through the Venturi part 10, the pressure in the negative pressure chamber is lowered, resulting in allowing the diaphragm 21 to displace upward against a spring 22 and consequently pulling a rod 19 so as to gradually open a flap valve 16 from its fully closed position (a) to its fully opened position (b). In such a manner as mentioned above, the air-fuel ratio in a region, in which the engine speed is low and the openings of the throttle valves are large, can be accurately controlled.