To provide a rotary throttle valve type carburetor with a fuel increasing mechanism easily operated, easily readjustable and capable of lifting a throttle valve lever in an idle position to supply an optimum highly-rich mixture.

A lifting member 28 is pressed with the finger to advance toward the throttle valve lever 8 in the idle position. A locking part 36 of an upper piece 34 of a tip part split in to two is engaged with an engaging part 45 of the throttle valve lever 8, and the throttle valve lever 8 is placed on an upper face 34A of the upper piece 34. An adjusting screw 43 is screwed into an internal threaded hole 41 to bend the upper piece 34 upward. The throttle valve lever 8 is thereby lifted to enlarge the open area of a nozzle hole by an integrally lifted measuring needle. A required and optional fuel increase can be attained by the bent degree of the upper piece 34 corresponding to screwing of the adjusting screw 43.