Happened to me once when a spider or something crawled into the air intake. Blasted it out with compressed air and no more soot. So check the air path not just the external control. You need more oxygen. This is good advice. We get lizards making themselves at home in the air intake around here all the time.
+1 on the lizards. When I lived in Florida I would have to do a lizard extraction from my burner before every use. There was one in there every time!
Here in Hawaii it is seasonal moth cocoons.
Old habits die hard. I clean all orifices on my burner before each use now. A cut piece of wire coat hanger and about three minutes of work, and then a brief wide-open blast of unlit propane to blow out debris insure efficient combustion.
I recently spotted a very nice
locally made propane burner at the local Ace Hardware. It has one burner that is made up of three separate concentric rings of flame ports. Each ring has three circles of flame holes. Each ring has a separate flame control valve on the gas manifold. It can be contolled from barely warming a tea kettle to lifting a hot air ballon. The frame of the burner is made out of ornate heavy steel, sits about 12" off of the ground, and has claw-foot feet like an old style bath tub. Awesome!
Every time I go into the store I stop and look at it. I feel like Elwood Blues drooling over the toasters in The Blues Brothers movie. I have a birthday coming up in a couple of weeks. I have let it slip that this burner is at the top of my birthday list..