it starts, runs and idles. it idles a tad erratically. as you put the engine under load (to move off, for example) it throws a wobbler with misfiring and pratting about, then smooths out again. sometimes it does this more than others. It doesn't make much difference whether it's hot or cold.
when it's messing about, it's losing at least 50% of the firing, for a brief period, sometimes more, occasionally it stalls. Once you get away from just-off-idle speed, it appears to run well, smoothly and with about the expected level of power.
It's recently been rebuilt with new rings and honed bores, and has recently had new plugs.
it's also recently had almost all the ignition system changed, thanks to a previous fault.
This morning, I put a different manifold/carb set on, which was known to be working well before. This has improved it slightly, but the fault's still there, just not as much as before. There's nothing obvious wrong with the mating faces or the inlet manifold gasket.
Adjusting timing or fuelling makes the normal sorts of difference to the general running, but doesn't really affect the problem.
The only thing I've not yet changed or tweaked is the plug leads. A set of magnecors is in theory on order, though no sign of it yet and I'll need to chase 'em about that soon.
Anyone think of anything I've not thought of?