An automatic control system for supplying a fuel-air mixture to an internal combustion engine through a Venturi structure conducting throttle-controlled incoming air to the intake manifold of the engine. Coaxially disposed in the casing of the structure is a cylindrical booster whose internal surface has a Venturi configuration to define a primary passage. Interposed between the booster and a ring having an external Venturi configuration mounted on the casing in an axially shiftable spool whose internal surface has a Venturi configuration to define a secondary passage having a variable throat between this surface and the spool. A tertiary passage is defined between the outer surface of the spool and the ring, incoming air passing through all three passages. An air-fuel dispersion is fed into the primary passage to intermingle with air flowing therethrough to create an atomized mixture which is fed through the variable throat of the secondary passage to further atomize the mixture which is then fed into the manifold. The air pressure difference between the inlet to the Venturi and the effective throat of the combined passages is sensed to produce a command signal for governing a servo motor in a closed loop arrangement to adjust the axial position of the spool to attain an optimum fuel-air ratio.