Engines with carburettors employ a mechanically operated fuel pump, mounted on the left-hand side of the rocker box and actuated by a short pushrod from the camshaft. The pump's design varies according to engine capacity, with these differences shown in the following operations. Engines with a fuel injection system use an electrically-operated pump in conjunction with a small auxiliary tank mounted underneath the car, adjacent to the differential unit. This type of pump begins operation as soon as the ignition key is switched on and can be heard 'licking' before the engine is started.