It runs, better, now that the carb settings are roughly correct. The ignition was retarded by about 25 degrees - as I discovered this morning. Pretty much at the limit of where the points plate can rotate for adjustment.
So, I set it all up as per the book and it refuses to start at all... just a cough when I release the starter button and a backfire occasionally.
It's a simple set up, one T mark and no mention of advance at all in the manual, so static setup at ignition retard.
It's hard to be precise, since my only Honda 250 experience was many Superdreams, but it seems breathless to me, and once running seems fine but then peters out and stops for no apparent reason... unless the throttle is blipped as it dies at which point it runs again for a few minutes before repeating the breathless dying cycle. Most odd.
Something, at least one of them, isn't right.