After installing a rebuild kit in an SU carburetor (especially an H-series) it is sometimes impossible to achieve a sufficiently lean mixture by manipulation of the mixture adjusting nut alone. Before resorting to the quick and make-shift solution of lowering the needle in the piston, examine these sketches carefully and compare the actual carburetor to them.
The proper solution may be effected through careful installation and assembly technique. The cork seal, located between the carburetor body and the cupped aluminum washer, fig. 1 and fig. 2, must be spongy and pliable so that it "cold flows" into the cup of the washer as the securing nut is tightened. When properly assembled, only a thin line of cork (0.5 mm) will be visible between the carburetor body and the top edge of the aluminum cupped washer. When properly assembled, the cork seal will
conform to the depression of the cupped aluminum washer and the entire jet assembly will be correctly positioned in the main carburetor body. This jet assembly positioning is critical if the mixture adjusting nut is to effect full mixture control from rich to lean. You will also insure that the mixture adjusting nut will provide enough jet travel to set the fuel/air mixture as lean as required while keeping the needle at its specified position in the piston.
Since the metal of the carburetor body is soft, do not, in your enthusiasm to squish the cork seal to its proper dimension, over-tighten the securing nut! Soak the cork seal in light oil (a day or two if possible) prior to assembly. The oil soak will make the cork seal pliable for easy assembly.