PURPOSE: To make it possible to atomize a fuel sufficiently even when the pressure of the fuel is low and to obtain an optimum air-fuel ratio at all time by a method wherein a fuel supply passage having a variable venturi, a variable throttle valve linked to the venturi and an air-fuel ratio adjusting mechanism therein is provided.

CONSTITUTION: A fuel supply section 9 is provided with the variable venturi 10 and a movable section 16 of the venturi 10 is displaced by a differential pressure responsive type motor 11 provided with a diaphragm 12 which defines a chamber 14 into which the negative pressure of air intake is introduced through an orifice 18 and a chamber 13 into which the atmospheric pressure is introduced through a passage 19. Further, a nozzle opening 20 is formed in the wall of the venturi 10 opposing the movable section 16 so as to receive a needle valve 12 serving as the variable throttle valve fixed to the movable section 16. The nozzle opening 20 is connected to a fuel pmp P through the fuel supply passage having therein an electromagntic valve 23 forming the air-fuel ratio adjusting mechanism M, and a fuel pressure regulator 24 operating in response to the negative pressure of air intake.