Abstract: An electronically controlled fuel injection system for an internal combustion engine which determines the pulsewidth of a drive signal to be applied to a plurality of fuel injection valves, each located within an intake manifold, which in turn determines the fuel injection amount per engine revolution according to the engine operating conditions, the air-fuel mixture ratio being smoothly changed from a lean air-fuel mixture ratio to a richer air-fuel mixture ratio when the engine load changes from a partial load to a full load. Therefore, fuel consumption is remarkably reduced without worsening the handling of the vehicle in which the fuel injection system is incorporated.