Abstract: A gaseous fuel control system for engines for delivering and regulating fuel and air flows to the engine and an electronically controlled rotary pressure regulator implementing fine adjustments in the pre-mixing pressure of gaseous fuel. Gaseous fuel from a high-pressure fuel tank is forced into a passage and through a series of pressure regulators during combustion. Inside a throttle body, butterfly plates respond to signals from an electronic controller to regulate the flows of air and gaseous fuel. The fuel passes into a stagnation chamber and mixes with the air flow through a series of radial mixing orifices in a common wall between the stagnation chamber and air passageways. The fuel/air ratio is coarsely controlled by the reaction of the butterfly plates to the signals of the electronic controller and is more finely controlled by electronic rotation of the rotary gas pressure regulator.