PURPOSE: To maintain always an appropriate air-fuel ratio, by carrying out the feed-back control of air-bleed for both main jet and bypass therefore in a variable venturi type carburettor.

CONSTITUTION: A needle valve 9 secured to the lower end of a piston in a variable venturi type carburettor piereces through a main jet 12. There is provided a passage 13 for bypassing fuel in order to comensate an inappropriate flow rate of fuel which is caused by the viscosity of fuel when the opening area of the main jet is small. A control device 31 openes a linear solenoid type control valve 30, excepting the times of idling, output and cooling of the engine while air bleed is fed, through an air-filter around the valve, from a passage 16 for the bypass passage 13 and from a passage 146 for a main system 14. The amounts of the air bleed is appropriately controlled in accordance with the outputs of idle port pressure sensor 33, intake-air pipe vacuum sensor 34, engine temperature sensor and oxygen sensor 36.