Referring to FIG. 2, there is shown an accelerator or governor means which enables control of the flow of fuel on the downstream side of the injector. In particular, in use, the governor means shown in FIG. 2 comprises a body 85 having a bore 86 which is in communication with the outlet 20 of the injector. The downstream end of the bore 86 is provided with a chamfered seat 87. Longitudinally selectively movable within the bore 86 is a governor 90 which has a complementary chamfered shoulder 91 which can close against seat 87 to completely close bore 86. The governor 90 has a shank 92 which extends into the bore 86 and is a close fit within the bore. The shank 92 has a groove 93 which tapers from the shoulder 91 to the upstream end 94 of the shank 92. The fuel can flow into the bore 86 along the groove 93 and between the shoulder 91 and seat 87 when the governor 90 is retracted longitudinally in the direction of arrow A. If the governor 90 is retracted only slightly from the seat 87, flow along the groove 93 is significantly restricted since the fuel must flow through the shallowest end of the groove 93 where the seat 87 meets the bore 86 at point 95. If the governor 90 is retracted further in the direction of arrow A, greater flow past point 95 is possible because of the deepening of the groove 93 towards the end 94. Thus the selective retraction and insertion of the governor 90 from and into the bore 86 enables control of the pressure in low pressure chamber 37 of the injector, which in turn, can control the stroke of the piston assembly 30, 35. If the governor 90 is moved to contact the shoulder 91 against the seat 87, the fuel flow through outlet 20 of the injector is prevented and this will hydraulically lock the piston assembly 30, 35 against movement by blocking the pressure relief path for fuel from low pressure chamber 37.