Abstract: An inertia governor mechanism applicable to a power unit providing a uniform preselected RPM when the power unit is subjected to variable loads. A pump means provides a constant supply of fluid at a constant pressure to a fluid circuit. A rotary disc valve in the circuit, having one or more axial openings therethrough and driven by the power unit to be regulated, varies the pressure of the fluid in the circuit inversely proportional to the speed, to provide a regulatory fluid pressure which acts on an expandable actuating cylinder to regulate the power unit with a constant speed.