An electrical motor controls the movement of a slider threaded over a threaded shaft of the motor. A sensor detects the position of the slider along the threaded shaft to control the movement of the motor which is also controlled by the operational state of the engine. The slider controls the position of a control sleeve fitted slidably over a fuel distributing plunger relative to the plunger so that the sleeve opens and closes a spill port provided in the side surface of the plunger in order to control the amount of fuel supplied to the engine cylinders.