Purely mechanical fuel injector for internal combustion engines utilizes a single plunger piston within a plunger barrel for fuel pumping, fuel/air mixing, and fuel injection functions. The plunger piston is reciprocated by a cam follower engaging a cam rotated by the engine. The fuel injector utilizes a fuel bypass valve to meter fuel to the fuel injector and/or bypass fuel to the fuel tank in response to the position of the air throttle valve on the engine air intake. Air is entrained into the fuel supply through an air fed mixing venturi.