An apparatus for controlling and varying the closing force of intake or exhaust valves for an internal combustion engine. Hydraulic system, responsive to a minimum high speed condition of the engine, is used to increase the spring rate of the valve closing springs. The hydraulic system also reduces the spring rate when the engine falls below such speed condition. A spring biased idler pulley is also used to maintain adequate tension on a belt, providing drive to the camshaft for the valves. Adjustment apparatus is associated with the spring biased idler pulley for controllably increasing the spring rate for biasing when the engine speed is above a minimum condition and for decreasing the spring rate when the engine speed is below the minimum condition.