A carburetor fuel adjusting device that facilitates control of the quantity of fuel that flows from the fuel chamber to an air intake port of a carburetor by making it possible for the user to adjust an adjustment valve within the limits defined by emission control regulations. The carburetor fuel adjusting device has a cap having two appendages to limit its turning in the lean mixture direction and in the rich mixture direction, and an engagement area to engage a valve extension of the fuel adjustment valves of a carburetor. The valve extensions are inserted through insertion holes of a retainer attached to the carburetor body. When the cap is retained by the retainer in a disengaged position, wherein the engagement area is not attached to the valve extensions of the fuel adjustment valves, the adjustment valves can be adjusted separately from the cap. The cap, however, can be moved forward to an engaged position wherein the engagement area of the cap becomes attached to the valve extensions. In the engaged position, the adjustment valves can be turned in unison with the cap within a range formed by the angle between the appendages which, when rotated, abut against stoppers.