To eliminate back clearance by screwing an incomplete synthetic resin screw shaft part of a fuel regulating needle valve into a screw hole of a carburetor body and to provide airtightness in the vicinity of the screwing part by press-fitting a shaft part of the fuel regulating needle valve into a shaft hole adjacent to the screw hole of the carburetor body.

The screw shaft part 14 with reduced outside diameter successively from a head part to a tip, a stem 13 and a tip needle 12 are formed on the fuel regulating needle valve 10. The screw hole 14 with reduced inside diameter successively inward from an end wall surface, a valve chest 5 and an outlet passage 4 are formed on the metallic carburetor body 2. By screwing an incomplete screw groove 14b formed by molding synthetic resin on an outer peripheral surface of a metallic body 14a of the fuel regulating needle valve 2 into the screw hole 14 of the carburetor body 2, deflection and looseness of the tip needle 12 of the fuel regulating needle valve 10 are suppressed. Airtightness in the vicinity of the screwing part is obtained by press-fitting the shaft part 15 of the fuel regulating valve 10 into a shaft hole 21 adjacent to the screw hole 14 of the carburetor body 2.