To provide a carburetor, especially for a motorcycle, of a constant- vacuum sliding throttle valve type that is simple in construction, smooth in operation and excellent in acceleration performance.

An upper cover 4 is coupled to the upper end wall of a carburetor body 6 or is placed over a diaphragm 5 to define an upper or vacuum chamber 14 and a lower or atmospheric chamber 13. To the diaphragm 5, a sliding throttle valve 26 is connected for vertical movement in an opening 17 orthogonal to an intake passage 33 defined in the carburetor body 6. On a support shaft 7 disposed in the atmospheric chamber 13, a biforked throttle value operating lever is coupled and a follower lever 8 is furthermore supported for idle rotation. The follower lever 8 is coupled at its tip to the sliding throttle valve 26. The throttle lever is mounted on its one fork with a journalled forced- control lever, and a lateral projection on the follower lever 8 is spring-loaded between the other fork of the throttle lever and the forced-control lever within the range between the idle position and preset opening level of the throttle lever, so that the follower lever 8 is rotated clockwise or counterclockwise.