The model radial engines all suffer from uneven air/fuel distribution to all of the cylinders. With a single carb you are cursed with a compromise for the adjustments. There have been various methods used to help aleviate the problem though. But those solutions can be quite complicated too. When the one guy did the separate OS engines, he left the carbs on each engine so each engine could be tweaked as needed. He simply bench ran each engine and adjusted it. Then bolted them all together. Then when he ran it, each engine was closely set already, so it wasn't difficult to dial them in then.

If you route the intake manifold plumbing for all of the cylinders to a single carb, now you have problems with the fuel falling out of the air in the tube and wind up with more problems such that the engine won't run at low speed well at all. You see similar problems with large car racing engines using long intake tubes like that too.