SU carburetors (named for Skinners Union, the company which produced them) were a brand of sidedraft carburetor widely used in British (Triumph, MG) and Swedish (Volvo, Saab 99) automobiles for much of the twentieth century. Originally designed by George Skinner in 1910, they remained in production through to the 1980s. Hitachi also built carburetors based on the SU design which were used on the Datsun 240Z and other Datsun Cars While these look the same, they are different enough that needles (see below) are the only part that fits both.
SU carburetors featured a variable venturi controlled by a piston. This piston has a tapered, conical metering rod (usually referred to as a "needle") that fits inside an orifice ("jet") which admits fuel into the airstream passing through the carbretor. Since the needle is tapered as it rises and falls, it opens and closes the opening in the jet, regulating the passage of fuel, so the movement of the piston controls the amount of fuel delivered.
The piston is moved up and down by the partial vacuum in the inlet manifold, depending on the position of the throttle plate, between the carburetor piston and the manifold.