Triumph Repair: 74 Spitfire, idles fine, weak acceleration, SU carb timing mixture conversion

Question
My car was running fine last year with the stock engine. This summer I installed twin European spec HS4's from Quantum Mechanics and 4-2-1 headers. I did a lot of research and I have everything plumbed correctly and kept the carbon canister and anti-run on valve connected. The carbs were completely torn down and cleaned. The needle valves and all gaskets have been replaced. It starts right up and is idling fine below 1000 rpm. It revs smoothly but when I drive it, you can tell right away the acceleration just isn't right. I checked the timing and it is BTDC but the mark is just past the end of the scale. This is how it has always been even before the mods. Adjusting it a little either way causes the idle to go down and die so it seems like this is best setting. I still have points in the distributor.
I am sure all my connections for the carbs are correct. The carbs themselves do not appear worn at the shafts. I adjusted the jets with a Colortune and it is blue with splashes of yellow. When you lift the carb pistons 1/16" the idle goes way down meaning it is lean but no matter how rich I make them, the idle still goes down, not up or stays the same. Engine temp is normal. Vacuum line from rear carb to distributor is good as is the vacuum advance diaphram.
Not sure what else to check or try.

Answer
2 things come to mind after your very thorough conversion:

1) check whether the advance of the timing is occurring.  With your timing light, rev the engine up to 3k RPM and see what happens to the timing.  It should advance significantly to far into BTDC (20-30D) when this is done.  If not, take apart the distributor and clean the weights, springs and advance/retard plate of the distributor, and make sure it is working and freely moves.

I'm not sure what you mean by "BTDC but the mark is just past the end of the scale".  Timing at idle, vacuum to distributor removed and capped, should be a 2-10D AFTC, depending on the engine/year/emissions.  But if you are 1 or 2d BTDC, that may be OK.  

2) HS4 carb needles may need to be changed for a better "taper" and mixture throughout the range, or so I"ve heard.  The 2 or 3 people I've heard, that have done this swap, have to change out the needles.  Joe Curto, of SU/ZS rebuild carb fame, usually knows what you need to do, needle wise, with the carbs to get them set correctly.

Sorry I can't be of more help.