There's no way that carb is too big. Minis with 1275cc engines run 45dcoe webers with 38mm chokes (from memory) which is way more carb on a smaller engine, and one that has less potential flow-wise with the intake ports such as they are.

What exactly is the engine doing that they can't seem to fix?
Without knowing more, completely off the top of my head I'd suspect that they might be going wrong with regard to emulsion tube specification. This particular issue requires it's own set of solutions on minis. I bring them up because more often than not they use a single weber to feed the whole engine (1 and 2, and 3 and 4 each share an intake port - 2 total - so there's no advantage to twin webers. I bring this up because a single sidedraught like this on a 4 will likely (the ports are separate but the intake manifold runners will be shared just prior to splitting at the manifold/head area, so the carb requirements will likely be more like a mini a-series engine, than what you'd run with twin webers on a dedicated manifold.

there's also a chance (for the same reasons) that the sort of acc pump setup would have to be more like a mini engine setup than for twin webers on a 1.5. The latter would (for the same setting) dump twice as much fuel on throttle opening, so you might have to ramp up the output if running a single dellorto (and I don't know how much scope you'd have to do that) - this alone could (if not addressed) cause a sizeable flat spot when flooring the throttle.

I'd be lying if I said I knew off the top of my head what emulsion tubes to try, but there is a book by Des Hamill which isn't 100% perfect, it needs more detail, but they do at least from memory have a few pages devoted to good 'starting point' choke and emulsion tubes for carbs where each throat is shared by 2 cylinders. there is some info in David Vizards mini/a series books that might help, but the ones I have are on loan to a friend. I can maybe get them back this weekend if I see him, and can post anything relevant. If a post doesn't turn up by this time next week, pm me incase I forgot.