Okay so here goes, try to make sense of my rambling.
This problem happens around 60-70% of the time driving.
I have an N12 elec. distributer and one of those MSD coils, and for fuel side I have a standard carb (DCH306 I think) with a modified standard air filter.

When you drive it, there is the usual flat spot all carby's seem to have, but when you try and cruise/take off from the lights, it hesitates badly.

When starting off from the lights unless you rev its tits off it dies in the ass as soon as you let the clutch out.

Last night on the highway (this isn't an isolated case either), it wouldn't for the life of it do a constant speed. You'd put your foot down, get it to 100, then let off the accelerator a little to maintain speed and it would just start to sputter its ass off and slow down, then you'd have to put your foot down to get it back to 100, then repeat.

Sometimes it can maintain a speed, but I can hear it misfiring and the tacho needle jumps up and down which leads me to think its an ignition problem.

I know it runs a little on the rich side because when it runs perfectly (and it does 1/10 times), it has a nice little decel burble/backfiring that I've grown accustomed to.

At first I thought this was a fuel problem as I can feel it surging at low speeds, but the tacho needle doing its little dance and the misfiring has me thinking otherwise.

The carby is an Ebay carby with a kit put through it and the distributer is I am assuming just a standard N12 one with the A series cog on the bottom and an adapter plate on the base to fit the Block.

sounds like the carby is running very lean could be blocked jets, anti dieseling valve not working or you have got an air leak somewhere (normaly that burble on decel gets louder the leaner it gets).

was the dizzy put on right e.g not a tooth off?

Its an early carb so there's no anti-dieseling valve, distributer is in correctly as I just did the bearings in the engine so that's dialed in. I might double check the carbs and blow a little air down the emulsion tubes to unclog anything. Tightened the two bungs on the back of the manifold as well to lessen the chance of a leak. Everything appears to be air tight, will listen closer when it's running tonight.

check your accelerator pump aswel to see if it squirts fuel into the carby when you hit the accelerator it should be a steady stream

and also look down the carby whilst keeping constant rpm (not pumping accelerator) to see if the accelerator pump is actually dribbling down the carby.

You mention the usual flat spot on carbys it could be a sticking power valve in the top air horn of the carby it should move very freely out it can cause a overly rich or lean mixture depending if stuck open or closed. the power valve is lightly staked into the top body and can be removed and polished with fine sand paper and wd40 (this has worked for me before)

The power valve will also play up if the top air horn is warped excessively which happens when the screws are over tightened you can check this with a flat edge.

These are just a few things i have encountered before and may be worth checking over just to be try eliminate a few things.

Thanks!

I just read that you have used a ebay/chinese carby

this is the same thing i am running and actualy runs very good on my a12 i took my power valve out and gave it a slight polish as mine was stuck in one position.

also the top gasket in mine was kinda cheap and fell apart as i took the top cover of so i replaced it with a spare i had.

i also changed all the jets using the genuine hitachi jets from my old carby as i felt that maybe the chinese jet sizes could be slightly off but they looked okay from just looking at them