An experimental investigation has been conducted to study the effect of free valve motion on the pressure transient and rain pump performance. In this study, the valve stem is attached to a leaf spring, which has a variable effective operating stiffness. The runs have been carried out at different initial valve openings for different spring stiffness. Fixed values have been used for the falling head of 2.5 m, and lifting head of 1.36 m. A computational model based on water hammer wave travel equations is used to verify the pressure transient due to valve self oscillatory motion. The governing equations are solved by the method of characteristics along the drive pipe with the appropriate boundary conditions at both ends of The pipe. The results reveal than the effective spring stiffness and initial valve opening have major effects on the ram pump performance. The stiffer spring, the maximum is the efficiency. Also the elastic spring, the maximum is the ram slow rate. Regardless of the spring stiffness. increasing the valve opening increases the ram flow rate. The design margins have been deduced defining the operational valve instability and/or ram pump performance.