A starting valve (100) for a fluid operating machine (200) for a vacuum system (1000). The starting valve (100) comprises: - a valve body (10) with an opening (17) to let a gas in; and an actuating piston (13) splined on a stem (16) free to slide in a guide (18A). The actuating piston (13), with a portion of the valve body (10), defines a chamber (30). The actuating piston (13) is subject to the action of a deformable diaphragm (15). A calibrated throttling hole (16B) is provided in series with a through hole (16A) in line with the stem (16) to establish a fluid communication between the chamber (30) and a fluid-dynamic line (500) of the vacuum system (1000). A shutter (20) is furthermore provided also splined on the stem (16). The shutter (20) closes a gas passage opening (10B) as a function of the pressure difference between the chamber (30) and the fluid-dynamic line (500). The calibrated throttling hole (16B) and the through hole (16A) establish a fluid communication between the space enclosed by a connection (25) and a space (22) defined by a cover element (23) positioned in the upper part of the valve body (10).