The invention relates to an actuating device that actuates engine valves, comprising magnetizing means, a spring having a fixed end connected to an adjustable stop and having a moving end connected to the valve (1). The adjustable stop comprises meshing means (26) that can interact with a control element (8, 10) whereby enabling the tare of the spring to be adjusted, and comprises an orifice (11) located between the adjustable stops of the two valves (1) of the same pair and serving to accommodate the control element (8, 10). The invention is characterized in that the control element (8, 10) comprises positioning means (12) that interact with complementary means (13) whereby making it possible to locate two stable positions respectively corresponding to the meshing positions with the two adjustable stops of the two valves (1) and to immobilize the control element (8, 10) in these positions.