Here we have another issue with SU H-Type carburetors that may result in fuel overflow. When the car sits long enough the fuel in the float chamber may evaporate and the float drops to the bottom of the bowl. This might happen fairly quickly with hot engine in hot weather. Sometimes the float valve lever may drop too low as a result of worn pivot parts or bad adjustment (bent arm). If the lever drops low enough, and the valve pin drops far enough, angle of the lever may bear against the valve pin causing it to bind in the valve body. This may prevent the valve from closing resulting in fuel overflow. This issue does not affect Gross Jets with the two small balls captive in the valve body.

You can cut a pair of thin slots in the heel of the arm to form a short tab that may be bent to limit amount of drop of the arm. Some new arms may already have these slots.

Float lever with Gross Jet. Click for larger image).