CD Stromberg needle adjusting tool - allen key type    P/N B20379

For adjusting top adjustment CD Stromberg - CD Zenith carburettors.

This tool is used on adjustable needle [top adjustment ] carburettors - allan key type

After removal of the damper unit from the carburettor top, the tool is inserted into the damper reservoir.
The central allen key locates in a socket in the reservoir base, while outer barrel locks into the air valve guide rod.
Rotating the allen key clockwise will raise the needle, richening the mixture. Anti-clockwise rotation lowers the needle, weakening the mixture.

The location of the outer barrel is important in order to prevent the air valve from rotating when the adjustment is being carried out, which would damage the  main diaphragm.   

It is advisable to use a CO meter during tuning operations to ensure the idle CO is set at its lowest possible level, which must at all times be less than 4.5%.

If it is not possibel to achieve a level of 4.5% within the range of mixture adjustment available, then some carburettor malfunction must be suspected, 
which should be investigated by means of a general carburettor overhaul.
If adjustment is to be undertaken without the aid of a CO meter, then idle CO can be reduced as far as possible by setting the idle to the lowest level that the engine will accept.