If a plastic float chamber, it could be warped. If so, dress it up with #400 wet/dry black sandpaper... the sandpaper wet down with mineral spirits.
Could be a float set wrong. If so, correct as follows.
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(Carburetor Float Setting)
(J. Reeves)
With the carburetor body held upside down, the float being viewed from the side, adjust the float so that the free end of the float (the end opposite the hinge pin) is ever so slightly higher (just ever so slightly off level) than the other end. And when viewed from the end, make sure it is not cocked.
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