Abstract: A fuel regulating method and mechanism for a rotary throttle (barrel-type) carburetor which prevents an end-user A/F adjustment that would cause increase in fuel quantity to a level in excess of a regulated value so as to comply with the exhaust gas emissions regulations. A cylindrical throttle valve having a throttle hole is disposed in an air intake passage of the carburetor body 12, the air flow is controlled by rotation of the throttle valve and the fuel flow is controlled by the position of a fuel regulating needle, attached to the throttle valve, relative to a fuel jet port of a fuel supply pipe in the carburetor body due to axial movement of the throttle valve. A bypass passage is provided within the carburetor body to communicate the throttle valve hole and fuel jet area with the air intake passage upstream of the throttle valve.