I'm not sure about the propane burners, but I have 3 of the 20 tip NG burners. These burners need a pretty good amount of pressure to function properly. I supply my stand with 3/4" pipe straight from the meter and go down to 1/2" pipe for the final runs to the burners. Worked fine with no soot. But when I installed the 3/8" Blichmann asco valves on my 1/2" pipe, I got a lot more yellow flame (not a lot of soot though). But restrict them more, i.e. turn the valve down too much, and I got soot like crazy.
It's a little counter intuitive, since yellow flames and soot are ordinarily indicative of a burner's being run too rich. But in the case of these burners, they need enough gas pressure for the jets to pull in air at the base of the jet as well as to off set the other jets to get a blue flame. Without enough pressure, they won't pull in the air and will run rich.