PURPOSE: To reduce the trapping of gas by increasing the flow rate of mixed gas in medium and low speed regions by providing a path varying part to function in response to a throttle valve to a suction pathway on the downstream side of the throttle valve.

CONSTITUTION: On an upper wall portion positioned at nearly the center of a suction pipe 3, a path varying part 5 is provided in such a way that its top portion 5a is positioned in the suction pathway 4. A throttle valve 2 and the path varying part 5 are connected through cables 9 and 10 to an acceleration regulator 11, and also can be each opened with respective intrinsic opening characteristics. Thus, the volume and cross section of the suction pathway can be increased or reduced according to the rotation angle of an acceleration grip 13, and the flow rate of mixed gas flowing in the suction pathway can always be kept within a constant range.