Manifold fuel/air flow

Just replaced a set of plugs in my 3500s to cure a misfire. Noticed that plugs 4,6,1,7 were sooted. This hints to me that one of my carbs is either a bit rich, or the choke might be not coming fully off on that carb.

Question is... Which carb feeds those 4 cylinders?

Also can anyone point me to an article about how the chokes work on a hif6? If I understand better, I have a better chance of working out my next steps.

Cheers Rover chums.