A fuel system is disclosed which comprises a fuel pump (10), a spill valve (11) and an injector (12) provided in a single unit. The injector (12) includes a valve member (21) biased towards a seating by a coiled compression spring (23). The spring (23) is engaged between a spring abutment (25) of the valve member (21) and a piston (27). The piston (27) is arranged such that the face thereof facing away from the spring (23) has fuel applied thereto when the spill valve (11) is opened in order to assist movement of the valve member (21) into engagement with the seating.