I think it's not enough air that causes the excess carbon build up. On some of the cheaper burners--Banyo, etc.-- it seems that they are not geared to provide the heat levels that we need for brewing. With my SQ14 it burned blue and clean as long as I wasn't pushing it for too much heat. During the winter or when I was trying to boil 10-12 gallons, it was tough to mix the gas and air to get it hot enough. So, I went with enough heat but ended up with the sooty build up on the kettle.
I finally got tired of getting soot on my clothes, pot holders and counters and upgraded to a Blichmann burner. Now I get enough heat and enough air, and voila, no soot.