PURPOSE: To restrict the opening degree of a fuel regulating needle valve which is not being brought in contact with a restricting member when the restricting member is fixed.

CONSTITUTION: A pin 34 projected outward in a radial direction is connected to the head part 37c of a fuel regulating needle valve 37. The serration hole of a cap 28 for restricting the opening degree of the fuel regulating needle valve 37 is fitted around the serration shaft part 24a of a boss part 24 which is projected from a carburetor main body 31 and allows screwing of the fuel regulating needle valve 37 in such conditions as the one side surface 28a of a notch C provided on the circumferential wall of the cap 28 approaches closest to the pin 34, and it is fixed by adhesive.