Abstract: A chemical pump actuator utilizes a variable pilot for operation of the piston power unit and an attached chemical injector. The pilot structure includes a pair of valve assemblies for regulating the flow of a power fluid into and from the piston cylinder. Pressurization and depressurization of the diaphragmmatic pilot, as regulated by a control fluid flow independent of the power fluid flow, operates the power fluid inlet and outlet valve assemblies. This pressurization and depressurization of the pilot is regulated by control fluid inlet and outlet valve assemblies, these assemblies being cam actuated by the piston upon completion of the power and return strokes. Exhaust of the power fluid from the cylinder is regulated by a throttle assembly with the length of the stroke of the piston determined by a stroke control assembly. Accordingly, the piston cyclic rate and stroke can be independently adjusted presenting a pump actuator capable of a finely tuned control of the chemical injector attached thereto.