Easiest way to check the needles is to remove the air cleaner, put your finger in the carb choke and lift the piston. Remove the damper piston from the dashpot first (black knob on top) and it should drop back easily.
If it binds, remove the damper and the dashpot (3 small bolts) and large spring, lift out the piston and check it is clean and the needle is straight. Depending on the age and type of SU there are other checks such as centralising the jet but it is best to have a manual for this (Haynes or maybe the one you can download from jclay's site).
Alan
Saab 9000 Stg 1
'81 TR7 DHC