Triumph Spitfire SU Carb setup
From Auto Wiki
All Triumph Spitfires use SU carbs as standard.
HS2 (1.25") Carburettors
Spitfires up to early Mk IV models were (usually) fitted with twin HS2 carbs. In standard form, these are fitted with "Red" damper springs, with DD, or BO needles.
If you have modified your engine in any way, you will probably want to use richer needles. With a lightly modified engine (free flow exhaust and K&N air filters) a good start would be to try Yellow damper springs with the richer H6 needles. This should improve the power output, especially noticeable in the 3500 to 5000 rpm range.
For anything more than light engine modifications, better results can be obtained by switching to bigger carbs, such as twin HS4 carbs fitted to later Spitfires.
HS4 (1.5") Carburettors
Pairs of HS4s carbs are fitted to later Mk IV and 1500 spitfires. For mildly modified engines, try using Yellow springs and the richer AAQ needles.
Wax stat jets are fitted to later 1500 models, and can give idling problems in hot conditions. A simple replacement can cause the problem, or use fixed needles with the aid of a conversion kit.