A fuel injection system for externally ignited internal combustion engines in which a fuel metering and distributing valve is controlled by an air sensing element disposed in the air suction tube of the engine and by structure which is adapted to alter the restoring force exerted on the air sensing element through the fuel metering and distributing valve. The noted structure includes a control pressure conduit, a pressure control valve connected to the control pressure conduit and a relief valve also connected to the control pressure conduit and connected to the fuel metering and distributing valve. The relief valve includes a sleeve valve and a throttle which is controlled by the sleeve valve. The throttle opens into a pressure chamber disposed on one side of a slide member of the fuel metering and distributing valve. This slide member exerts the restoring force applied to the air sensing element.