The position of the forked in the float chamber controls the float height and therefore, the fuel level in the jet. The Workshop Manual for the Series “E” specifies a distance of ⅜” between the fork and a line across the float chamber lid when the needle is seated. This is achieved by using a section of ⅜” diameter bar across the lid.
When purchasing a new needle and seat for the carburetor (the ones in the carburetor bore no resemblance to the originals), I was discussing SU carburetors generally with John Cheal of Wilson Carburetor Service (no relation), who insisted that the correct distance of the float fork was 7/16″ and the Manual was wrong.
So who is correct? I set my float at the 7/16″ level and, with the new needle and seat, am achieving smooth running and good fuel consumption (average of 40.4mpg for the 2400 miles of the Caloundra Rally trip; moveover, inspection of the spark plugs shows that light grey-brown deposit. So, I conclude that the carburation is fine, and, therefore, my vote is for Mr. Cheal.