PURPOSE: To restrain the thinning of mixture at the time of the middle and high opening operation of an engine by opening a fuel quantity increase passage by actuating a control valve by means of a control signal with each throttle valve opening being more than a fixed opening and also within the range of determined in take pipe inside negative pressure.
CONSTITUTION: Each carburetor 1 at which the effective opening area of an intake passage 2 is conducted mechanically with opening/closing control by means of a throttle valve 9, and a throttle valve opening sensor 27 that is arranged at a fuel quantity increase passage 20 and detects the opening of each throttle valve 9, are equipped. Also, fuel quantity increase distribution passages 20A-20C or like that are made to branch off from the fuel quantity increase passage 20 downstream of a control valve 21 and are connected to the intake passages 2 of respective carburetors 1, are equipped. The fuel quantity increase passage 20 is made to open by actuating the control valve 21 by means of a control signal from a control circuit 30 with each throttle valve 9 opening being more than a fixed opening and within the range of intake pipe inside negative pressure that is determined within the range of the opening of each throttle valve 9. As a result, the thinning of mixture at the time of the middle and high opening operation of an engine can be restrained.
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