This device is used on adjustable needle (top adjustment) carburettors of the later CD type. After removal on the damper unit from the carburettor top, the tool is inserted into the damper reservoir. The central slot locates in a socket in the reservoir base, while the outer barrel locks into the air valve guide rod. Rotating the Allen key clockwise will raise the needle to enrich the mixture, anti-clockwise rotation will lower the needle. The location of the outer barrel is important in order to prevent the air valve from rotating when the adjustment is being carried out, thus damaging the carburettor main diaphragm.