First get an adjustable regulator for the propane tank(try propane company, farm supply, hardware store).Add a gauge that has a 20 to 30 PSI scale. This will allow you to get more or less propane w/o having to monkey around with the orifice. Use a piece of fiber or old kiln shelf to adjust the vent (flue) where the hot gas exits at the top. Make sure there is plenty of secondary air coming in around the burner. Open the regulator until you can barely detect flame coming out the top, then turn the regulator back until the exhaust is clear. If the kiln gets hot, great! If not you can close the exit flue some.. Just remember the initial load can take considerably longer than subsequent ones. Adjusting BTU's up or down and how much flow there is should get you to a reliable firing.
Do you use the regulator to adjust the weed burner output (while keeping the needle valve of the burner fully open) or it makes sense to keep the regulator at some setting (the highest?) and regulate the burner by the needle valve?
I'm at the planning stages of making a racu kiln (most likely, a converted old electric kiln), and am going to use my 500,000 BTU weed burner although I understand it could be on overkill for a 4-5 cu ft chamber. By the way, will a 0-30 PSI regulator/gage sufficient for 500K BTU or I should go for a 0-60 PSI one?
EDIT: I checked some references and spoke to a couple of people. Red Dragon 500,000 btu weed burner needs optimal pressure of 30 psi (and the permissable working range is up to 90 psi), so I guess I'll go with a 0-60 psi regulator to have a healthy margin. I can always dial below 30 psi if needed.
And using the needle valve to regulate the flame while keeping the regulator at a set point was another recommendation,