To perform the smooth starting of an engine and smooth warming-up operation continued thereto by a method wherein a movement amount in an axial direction of a throttle valve lever is caused to approximately coincide with the cam lift of a cylinder cam member regardless of the magnitude of the opening of a throttle valve.

A rotary throttle valve type carburetor to control a fuel amount through rotation and axial movement of a throttle valve is formed such that a plurality of cam surfaces 60a and 60b the heights from an axis of which are different from each other are arranged on a cylinder cam member 45 having a manual starting operation lever 45a, and a throttle valve lever 21 is axially moved through rotation of the cylinder cam member 45. The under surface 62 of the throttle valve 21 making contact with the cylinder cam member 45 is gradually inclined downward in an opposite direction to the acceleration direction of the throttle lever 21.