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[T3] Jerky acceleration

Last Sunday the car seemed to be running almost perfectly, but by Tuesday, the jerkiness was there again really badly. I thought I'd found the cause when I removed each plug lead in turn and #4 had no effect. I found the 'new!' plug cap was corroded on the screw where the cable screws in. I fitted an old lead, no change. Changed the plug for a new one, gapped at 28thou, no difference. Checked valve clerances and both were 'a reasonably tight' 6thou. I opened them to 7thou and I found that then #4 cylinder gave the same power as all the others. But the car still jerked as it drove. No changes to the carb settings made any difference.
I checked the timing and it was back at 10 degrees. Reset it to 12.5 degrees at 900rpm (which is right for the dizzy) and it ran better again, the jerkiness was back to the more minor problem I was asking for help with last week. As I've now bought a new timing light with advance, I swapped the dizzy back to the 009 and set the timing to max 32degrees (it's 12.5degrees at 900 too) but it ran just the same. So the dizzy is not the problem, but any slight retard of timing makes it miss really badly. I've never known a car to be that sensitive to timing.
I tried reducing the accelerator pump stroke on one carb (by redrilling the rod so the pin could be moved back) and it seemed better. I've not had time to do the other yet. I'm now wondering if the mixture is too rich on low speed acceleration and maybe flooding causing cylinders to miss, and by reducing the pump volume, it's helping not to flood them out? I'm so desparate that I could convince myself of anything now!!!!

Oh, I checked the tickover carb balance too, so it's not that, although it's possible it's slightly out on higher speeds as i didn't get time to check that. (But the linkages don't pull on the carbs at all).

Any further help or comments from anyone?

Rgds chris

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