A liquid fuel system for an internal combustion engine includes a fuel supply line, a fuel pump connected to the supply line and having an outlet to a carburetor or a liquid fuel injector. A mechanical contact between the pump and a component of the internal combustion engine transmits a cyclic motion from the engine to the pump to cause the pump to pump at a cyclic rate determined by the engine. The pump is a variable displacement pump, so the output flow rate of the pump is diminished by a sufficient backpressure at the outlet of the pump.