Grub - Without the numbers, difficult to do more than guess, but a little background information:
Carter used the AFB on most larger marine applications from about 1960 to 1967. These (mostly) have the weighted airvalve which is BELOW the secondary venturi, but above the secondary throttle plates.
In 1968, the AVS was used on some of the smaller V-8 marine engines, typically from 307 to 350 CID, while the AFB was retained on the larger V-8 engines. The AVS has the spring-loaded airvalve at the top (many call it a second choke valve).
Chris-craft used a number of different AVS units. These did not work out well (mostly, the AVS did not work well on any application), and by 1973 this bad idea was history. AFB's were again used on V-8 marine applications.
Carter offered a Coast Guard approved AFB as a "service" unit to replace the AVS, when needed.
I was always of the opinion, that a twin-engine boat should replace both, if one had problems, but not all owners agreed with me. I still believe that a twin-engined anything should have absolutely matching engines.
Assuming that the carbs in question are either original or a correct service replacement, either should (if in proper tune) run a single engine well. The fact that one does not indicates (A) either the wrong carb, or (B) a correct or service unit in need of work.
As to your comments on Q-Jets, I love the Q-Jets on passenger cars (almost as good as the Carter TQ's); but do NOT like them on trucks larger than 1/2 ton, or boats. Much prefer a square-bore carb for either of these applications.
Jon.
