A motor vehicle fuel system operable selectively to supply metered quantities of petrol or LPG in its gaseous state to a carburetter induction passage. The LPG system includes a pair of digital injection valves in a body having a common inlet, a common outlet and an outlet chamber into which the gaseous LPG is injected alternately by the two valves. The metered pulsed outputs of the two valves collected in the outlet chamber is conveyed as a single gaseous fuel supply to the carburetter induction passage. The injection valves can be located spaced from the carburetter whereby to avoid excessive heating. An electronic control unit including a microprocessor and a data store matrix responds to certain engine operating conditions, including engine speed and engine loading, and controls alternate operation of the valves in synchronism with the engine ignition firing sequence. Under certain conditions one of the valves may open before the other closes.