A carburetor fuel mixture adjustment assembly has a threadable holder which suppresses rotation of a needle valve body disposed within an elongated receptacle defined within a carburetor body. The threadable holder is rigidly engaged within a threading chamber of the receptacle. During assembly, the needle valve body is extended through a bore of the holder as a threading portion of the valve body rotateable cuts an interior threading groove into the threadable holder. The threadable holder is held rigidly within the threading chamber by the carburetor body. A sealing holder is disposed within the receptacle axially inward of the threadable holder and is prevented from rotating within the receptacle by the threadable holder. A stem holder portion of the sealing holder assures a needle portion of the needle valve assembly remains concentrically disposed within an outlet orifice of a fuel chamber of the receptacle. A shank holder portion of the sealing holder engages radially between a shank chamber wall of the carburetor, which axially defines a shank chamber of the receptacle, and the shank portion of the needle valve body disposed concentrically to and axially inward of the threading portion. Engagement of the shank holder portion prevents air from entering through the receptacle and into the sub-atmospheric fuel chamber, thereby assuring stable fuel flow through a fuel passage intersected by the fuel chamber and stable engine operation.