PURPOSE: To prevent unevenness in the fuel injection characteristics by providing adjustability for the neg. pressue around a fuel nozzle, which is open to the internal face of suction passage, facing the bottom of a sliding throttle valve during low degree of opening.

CONSTITUTION: A communication line 60 is formed between the upper space 59 of a sliding throttle valve 12 in a throttle valve guide cylinder 13 and the upper stream end of the suction passage 14, i.e. the portion of pneumatic horn 19. At the bottom 12d of this sliding throttle valve 12, a recess 67 is formed facing a recess 28 in said suction passage 14 so as to adjust the velocity of suction stream in the neighborhood of main fuel nozzle 32, as well as an air passage 68 in communication with the above-mentioned space 59 and an air screw 69 screw- fitted therein are installed so as to adjust the rate of flow of the air. Thereby the neg. pressure around the fuel nozzle 32 during low degree of opening of the sliding throttle valve 12 can be adjusted to a proper level, and now the fuel injection characteristics are stabilized.