yep. couldn't explain before because i was working (in theory)
When you put your foot down on the accelerator reasonably hard the engine needs a richer mixture because a whole lot more air rushes into the engine all at once. Many carbies have an accelerator pump or accelerator jet that gives an extra squirt of fuel when you put your foot down so the engine gets enough petrol.
In an SU carby, on idle, the piston/plunger sits in the way of the incoming air so that only a small amount of air can pass underneath the piston. The piston is held up by the vacuum that the engine is producing so higher rpm lifts the piston higher. The spring helps hold the piston down too. The air picks up fuel as it flows over the jet due to the venturi effect (you should be able to find this on wikipedia if you don't know what it is).
The way that the SU achieves the accelerator pump effect is by having a dashpot on the carby. This stops the piston from moving in a hurry. So when you put your foot down, more air starts rushing into the engine, but the piston will only move up slowly. You then have more air rushing through the gap under the piston, which will enhance the venturi effect and more fuel will be picked up, richening the mixture.
If the consistency of the oil in the dashpot is wrong or there isn't any oil in there and the piston goes up too quickly then you will get a flat spot in acceleration when putting your foot down. If the oil in the dashpot is too thick then the piston will fall slower and won't have enough time to fall back down when you change up gears, so you will get a flat spot on each gearchange.
The spring effectively sets the position of the piston on idle - the weaker the spring, the higher the piston will sit, and vice versa. If the spring is too weak (as it was on my car) then the accelerator pump effect may be too weak because the piston may be sitting too high up and the sudden rush of air when you put your foot down may not provide a large enough venturi effect.
The accelerator pump also depends a bit on needle choice so everything is nicely interrelated and it can take a bit of messing around to get it right. It isn't all that hard though - just change bits and pieces and see what happens 
sorry if I didn't explain well. if you have pulled apart an SU carby before you should be able to work out what i am talking about, otherwise you should be able to work it out if you can find an exploded diagram
And now you see why i didn't want to explain while i was at work 
I don't know about the fuel pump kits either because i have never needed one, but i don't know if they are repairable anyway - would probably just be cheaper to find a second hand one
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If the rest of the carby is functioning correctly then it should be ok, i had adjust mine a lot towards the rich side as well. It will depend on what needle you are using.
