If you find someone to rebuild your SU's have them install new bushings in the throttle body and new throttle rods, rather than just oversize throttle rods. The throttle bodies in HS6's tend to wear eliptical. While oversize replacement rods help, this is a short term solution in most cases. I have looked at over 100 SUs from volvos during the last 25 years, and have not seen many carbs that were adequately repaired with just oversize throttle rods. Also it is important to measure the diameter of the piston, cover, dashpot(damper piston), and dashpot bore, with micrometers when doing a complete overhaul. Often these are worn out of spec and merely replacing throttle rods, flaps and needles will not give you originial performance. Many folks aren't this thorough, but if you have driven a car with new carbs vs one with sort-of rebuilt ones, you know what I mean. I have always done my own rebuilding at my in-house machine shop.

Blaine England, Volvos galore incl: 1956 TP21, 1958pv 444, 1959 pv445