11 a valve 12 and a valve 13 said three elements being arranged for work in parallel the valve 12 comprises a movable member 14 held by a spring 15 against a seat 16 the tension of the spring 15 is chosen so as to maintain the fuel pressure in the pilot manifold 8 at the value required for steady combustion of the fuel delivered through the pilot burners 9 the flow area of the jet 11 is chosen so as to enable fuel to prime the main manifold 5 before the valve 12 opens the flow area of the valve 13 is much greater than the flow area of both the jet 11 and a valve 12 the metering element 4 has a needle 17 and a body 18 the needle 17 is connected through a mechanical linkage to a control unit 19 which comprises a bellows 20 a chamber 21 and a chamber 22 the chamber 21 connects with the inlet to the engine compressor not shown the chamber 22 communicates via a pipeline 23 with a chamber 24 arranged to receive air from the engine compressor through a pipeline 25 provided in one of the walls of the chamber 24 is a hole 26 adapted to open said chamber 24 to the atmosphere the flow area of the hole 26 is regulated by a valve 27 which is controlled by a centrifugal speed sensor 28 of an engine speed governor the latter is not shown the metering element.