If you have a stock B18 engine, my understanding is that you start with ZH needles which are the most common on HS6 SUs.

Worked for my setup.




From: http://home.earthlink.net/~goodspeeds/NEEDLES.HTM
SU HS6 Carburetor Needles used on Volvo B18 and B20 engines.
Dimensions in inches. Drawings not to scale.
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Replacing the needle in an SU carburetor with a thinner one will result in a richer fuel/air mixture. As you can see from the picture, the KD needle provides the richest mixture, and the ZH the leanest. Always use the same kind of needle for each carburetor in multi-carb setups. Stock needles are best for stock engines in street use. To purchase SU carburetor needles in the USA, call Joe Curto, 718-762-7878.