OK, got carbs from the bloke on eBay. one appears almost brand-new (although it has been used and has been apart, judging by the evidence.
put it on Edward... makes almost no difference. Now got more or less perfect idle, but still won't run right above idle, and at high throttle openings misses often but erratically and makes much bang and pop in exhaust, which seem to imply fuel getting through.
Summary:
Fuel pump almost new, and delivers mucho petrol if the pipe is pulled off when it's running, not suspect.
Distributor (inc cap, condenser etc.) new.
Inlet manifold intact, no sign of big hole under the carb (was one thing that needed eliminating).
No obvious air leaks.
cranks normally and starts on idle with no trouble. Idles nice and smooth and can be slowed down below normal idle speed without stalling. ergo nothing major wrong with compression or suchlike
Crankcase ventilation thing removed and blanked off, to eliminate it.
Coil changed for known good one from another motor.
temporary supply with a bit of wire from battery to coil didn't affect it either.
Plugs changed for other ones (not new) but with no effect. Leads ditto, no effect on fault symptoms.
Replacement carb opened up, float level correct and all squeaky-clean inside
- specifically, no crud blocking the main jet. It'd have to be something quite noticeable to give the observed symptoms. Carb really does look almost new. Also, fault is the same (bar for better idle) as with the Weber carb on - and I think if I'd slowed the idle down on the weber it'd be OK - speeding the idle up makes it erratic.
Moving ignition timing 'til it's obviously retarded made no difference, so not ignition timing - it'd have to be miles out to give the observed symptoms, anyway, and it'd not idle properly. Vaccuum pipe appears to suck. missing vacuum advance doesn't, in any case, produce symptoms as severe as these, IME.
Engine earth must be OK, or it'd not crank properly.
Apart from trying new plugs and leads, what have I missed? I probably will try new plugs and leads, but I'm not confident.