engine running rough

[Chris@Nitron]

Hi guys, i have just rebuilt a caterham superlight R with a slightly modified k-series engine.
when i start the car it doesnt like to idle very well and when the revs are increased it seems a lot smoother, i tried to drive it up and down the road, but the engine didnt being under load, when i floored it, it felt as if the engine had a limiter because it was missing so much.
ive been through the wiring, but cant think what the problem might, any help would be much appretiated.
Thanks Chris