PURPOSE: To secure a favorable air-fuel ratio at all operating ranges, by setting up a slide valve and a butterfly throttle valve, as a variable Venturi, in a suction passage in turn, while opening a low-speed system fuel nozzle in and around the throttle valve, a main fuel nozzle just under the slide valve and a primary duel nozzle in position between both these valves, respectively.

CONSTITUTION: In time of low-load operation that makes a butterfly throttle valve 7 low-opening, a quantity of fuel commensurate to opening of the throttle valve 7 is discharge out of a low-speed system fuel nozzle 19, and fuel control is performed in a highly accurate manner. Next, in order to operate an engine at medium- and high- loads, when opening of a slide valve 6 is set to the medium-and high-opening, suction pressure on a main fuel nozzle is constolled according to load by means of a variable Venturi function whereby a fuel quantity suitable for these medium- and high-loads is discharged out of the main fuel nozzle 10. Likewise, in time of transition from low- load to medium- and high-load, since suction pressure between the throttle valve 7 and the slide valve 6 grows larger than that at the downstream side of the slide valve 6, such a quantity of fuel as making up for a portion of shortage in the fuel discharge out of the main fuel nozzle 10 is made to be discharge from a low- and medium-system primary fuel nozzle 28.