Everyone has an opinion on SU needles. The best thing to do for an unmodified engine is to use the needle originally specified. Volvo used different needles for different applications (ok, so a DUH) but they also used different needles for different air filters (makes sense when you think a bit).
Right.
In order from richest to leanest (also generally the steepest taper to the end)is:
DX; SM; ZH; TZ; KA; KF; KG; KE; KB.
The "K" series needles (with the exception of the KD but that was a spring-loaded needle)are emissions and low-volume needles (low engine volume, usually low speed).
The KA especially seems only suited to teenager cars or stationary engines.
These can all be acquired (along with pretty much anything else you may desire of SU's) from Burlen Fuel Systems (www.burlen.co.uk). They can also provide the original needle spec.
I plan on using the DX needles in my upcoming B20.

Mike!