Abstract: The priming system for an internal combustion engine adapted to operate on a gaseous fuel includes a normally closed, solenoid-operated priming valve electrically connected to the engine ignition circuit through a selector switch and the normally closed contacts of a vacuum switch when the selector switch is an "automatic" position. The priming valve controls a small flow of the gaseous fuel into the air intake of the engine carburetor. The vacuum switch has a pressure-sensing chamber which is connected in fluid communication with the engine intake manifold. When an operator moves the engine ignition switch to a start position with the selector in the "automatic" position, the priming valve is energized open to admit a small flow of the gaseous fuel into the carburetor air intake for priming. When the pressure in the engine intake manifold decreases below a predetermined level during engine cranking, the vacuum switch opens to deenergize the priming valve which closes to terminate the flow of priming fuel.