It's very similar to my own (an HB) in that I have a late (1984) low-mileage Chevette 1256 engine, but with the Zenith 30IZ carb, plus Bosch Dizzy too, and electronic igntion (Powerspark) too.
I'm Clutching at straws really and without experiencing it hands-on, at a disadvantage.
These are not the most precisely engineered engines, and anything from worn camlobes to weight differences in pistons and other moving parts could cause inherent imbalances manifesting as rougher running. Is there any variation in behaviour as engine temperature changes? Tappets are best a bit on the wider side rather than too tight, even if it means it's a little bit noisier while warming up.
The only outright failure I had with this setup, once the carb was attended to with new diaphragms, float-valve etc., and keeping the accelerator pump linkage setting on the summer setting all the time, never winter as it was too rich, was with the electronic ignition unit's wires inside the dizzy having the insulation worn away by contact with either the shaft lobes, underside of the rotor, or the trigger ring (most likely) causing a misfire when near flat out climbing a hill due I surmise to movement of the advance mechanism eventually causing one or other of the units exposed wires to contact ground. These wires too through flexing and the internal strands failing, maybe through getting pinched could intermittently break their connection especially as the advance mechanisms are in constant motion one way or another sometimes quite violently, as revs and throttle opening change.
You might try moving the ignition unit's trigger rotor round one quarter turn to the next lobe, as it could be gone faulty from friction against something, check too that there's no swarf, iron filings or anything sticking to it. It might move the problem to the other pair of cylinders, but would need a good run starting with clean plugs to determine any change. I've assumed the blackened plugs were sooty, rather than oil-fouled.
It might just need an 'Italian Tune-Up'. A damned good fast-blast run and sustained wide-open throttle on a long-straight, after getting it fully-up to operating temperature. I never really got satisfactorily smooth running until I started using Tetra-Boost tetra-ethyl-lead additive, just a little, less than recommended to create true four-star which really got it back on song. It would be making it more rich still, but can compensated for with carb tweaks and running near enough (just half a degree less) original specification static ignition timing.
I'm out of ideas, hope someone else can chip-in too. Have to run, do keep us posted!