Abstract: An electronic fuel injection system operable as a bolt on retro fit replacement for a wide variety of carburetors is disclosed. The system includes a throttle body-injector assembly which, by means of an adapter may be bolted directly to stock intake manifolds using the carburetor mounting bolt holes in the manifold. The throttle body passages and injectors are designed to meet the fuel and air delivery requirements of large displacement engines and an electronic control unit which controls the solenoid actuated injectors in a duty cycle operation is provided with externally accessible adjustments by means of which the system may be adjusted to tune the rate of fuel injection to the fuel delivery requirements of engines of displacements much smaller than the largest displacement engine within the systems capability.