Downstream of the flow restriction means 135 there is a non-return valve 140 which functions to maintain a minimum back pressure determined by the force of spring 141 which is a function of the spring itself and the position of adjustable seat 142 for the spring 141. At low idle speeds of an associated engine, the valve 140 determines the minimum back pressure. At higher engine speeds, the valve 140 remains open substantially all of the time.

The system shown in FIG. 4 also provides a controllable damper means 150, illustrated more clearly in FIG. 6. The damper means 150 includes a movable damper member 151 illustrated as a damper disc mounted in a damper chamber 152 which is in communication through duct 153 with the intermediate chamber 121. The damper disc 151 yields resiliently upon an increasing pressure in the intermediate chamber 121. There is an adjustable stop member 155 which is adjustable by means of set screw 156 to enable selective setting of the limit of resilient movement of the damper disc 151. By adjusting the position of the stop member 155, the idle setting or speed of an associated engine can be effectively controlled. In particular, a relatively large gap between the stop member 155 and the damper disc 151 enables a larger stroke of the piston assembly 30, 35 before the other flow limiting means or pressure relief limiting means become effective, thereby enabling a higher idle speed to be set.