A regulating device used with the fuel injection system of an internal combustion engine, for regulating the proportions of the operating mixtures of the engine. The regulating device is embodied as a pump which is incorporated into the existing fuel injection system. The pump has a chamber within which a piston is displaceable. This piston is coupled with the fuel rate adjusting member of the fuel injection pump of the fuel injection system, and is displaced in accordance with the adjusted position of the fuel rate adjusting member. The pump is also connected with the throttle plate of the fuel injection system located in the induction tube of the engine. By reason of these connections, rapid adjustment movements of the fuel rate adjusting member can be effectively translated to a proper adjustment of the throttle plate without the delay found in presently known regulating devices.