To provide an improved fuel adjustment device for a rotary throttle valve type carburetor and a method for fuel supply adjustment.

The rotary throttle valve type carburetor is provided with a piecing air duct in which a throttle valve moves between an idling position and a throttle opening position, a fuel quantity adjustment needle for changing the size of an injection port of a fuel injection passage, and a bypass air duct. An outlet of the bypass air duct is connected to the piecing air duct at the idling time of an engine, and closed by the throttle valve when the throttle valve moves from the idling position to the throttle opening position. In setting for idling, an upper stream end of the bypass air duct is opened to a bypass air source to allow the air extraction and to set a fuel ratio of fuel mixed air to an allowable maximum value. In addition, the fuel quantity adjustment needle is raised to enlarge the opening of the injection port.