Before you do anything else, check you have a good spark on every plug. I had a similar problem and it turn out to be an intermittent misfire on two cylinders, thanks to duff ignition leads. Just hook the strobe to each lead in turn and make sure you have a good consistent spark.
I have mine at about 8 degrees, like yours the max I could get was 10, but that is because my engine is made from many different engines and I guessed the correct position for the dizzy. Once I had it close, I wasn't moving it. The manual will be for 4 star, I don't know how unleaded effects it, but it should run smooth at the setting you have.
Don't think the vacuum advance will make any difference at idle speed, but I don't see any advantage in taking it off.
My bet is an ignition fault, if not that it must be a fuel issue or an air leak. Unless you have bad compression, but if it is a new engine, hopefully we can eliminate that. You did put the breather system back? Mine was not happy when I forgot to refit the breather pipes.
Thanks for your thoughts. I don’t have the breather connected currently as there’s nowhere to fit it on the dellorto manifold, something I need to look in to.
I was curious about the spark, ran out of time yesterday to investigate, I have new plugs, leads, cap, rotor and electronic ignition. I’ll try the strobe on various cylinders and I’ll grab my insulated pliers designed to remove leads whilst running and pinpoint weak cylinders. Worryingly there was fuel dripping from cyl 3/4 carb, so may be a problem there, need to establish correct pressure too, not adjusted it yet but was .2 bar, bit high maybe. Also I have my original cap and leads somewhere, so may donor them over. Most annoying the dizzy doesn’t sit in the centre of slots! The book must be wrong!