Abstract: A fuel pump for supplying fuel to an internal combustion engine includes a cam ring movable angularly to determine the timing of injection of fuel. The cam ring is moved by a piston against the action of resilient means by fluid under pressure from the outlet of a feed pump the output pressure of which varies in accordance with the speed at which the apparatus is driven. A second piston is provided and this is moved by the aforesaid fluid pressure against the action of further resilient means in a direction away from the first mentioned piston. The second mentioned piston is subjected to a fluid pressure assisting the action of the further resilient means, the second fluid pressure depending upon the amount of fuel which is being supplied to the engine.