Series 3 Vacuum advance

1972 Series 3, 2.25 petrol engine with new Weber Carb and electronic ignition, both fitted back end of last year by a garage. Running really well, no problems at all. All of a sudden while doing about 40mph, cough, splutter, terrible misfiring. Made it home though. Couple of days later, drove it to a mates house... barely made it.
Checked sparks, HT leads, Distributor not loose, fuel getting there, air hoses intact. Because it seemed to happen so ‘out of the blue’ I suspected either a jet blockage in the carb even though we have an inline filter fitted. Either that or the timing was out. Checked the carb (because I’m not confident with messing about with timing) cleaned some bits out, thought that would be it. Nope.... same as before, running as rough as a badger. Mate disconnected the vacuum advance from the carb and bingo, running a whole lot better, just one small stutter while driving it home (albeit taking it easy). Does some googling, some suggest it could be the Advance diaphram in the dizzy. Would this be likely in a new unit? Any ideas please or further tests I can carry out to diagnose this.?
PS. I am new to this game so any help offered, can you keep it simple so the stupid (ie. me) can understand.

Could also be the mech advance anf retard failed to full advance. If it has one. Take the plate off and look underneath.

rob1miles said:

Could also be the mech advance anf retard failed to full advance. If it has one. Take the plate off and look underneath.

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You might have a problem with one of your bob weight springs, one of mine snapped (god knows how) and it ran a rough as. I've fitted a new one but not yet re fitted the dizzy.

Dizzy fitted, running great.

Highlander, I had a similar problem. I replaced all the leads, coil, contacts and plugs and made not difference. I was about to dive into tuning the carb when I realized that the choke cable had jammed, I replaced this with a solid wire one (ie not relying on the return spring) and it was sorted. I note that you have checked plugs and I presume they weren't oily? But I mention this experience just in case. Always best to eliminate the simple stuff!