To provide a starting fuel increasing mechanism for a rotating throttle valve carburetor which allows starting of the engine, easily eliminating the need for troublesome starting operation and the revolution at a start not increasing high.

In a rotary throttle valve carburetor, in which a throttle valve lever 7 is fixed at an upper end of the rotary throttle valve 6, an idle adjustment screw 48 is made to contact the side of the throttle valve lever 7 to adjust the idle opening, and the throttle valve lever 7 is moved upward, to increase the area of a nozzle port 56 of a fuel nozzle 9 for increasing the fuel amount at the start of operation, a cam part 50 to rotate the throttle valve lever 7 to the opening side of the throttle valve, when the throttle valve lever 7 is moved upward is provided at the side face of the throttle valve lever 7, where the idle adjustment screw 48 makes contact. In this way, the throttle valve 6 is opened slightly, when the throttle valve lever 7 is moved upward to the start position, amount of air is slightly increased in addition to the increase of the fuel, so as to smoothly drive the engine after it has started.