LP is heavier than air and will burn dark orange and produce soot.
Does this sound like your problem? Don't worry we can fix that. A LP air mixture valve is not so much a valve as it is a special fitting. This fitting is normally connected to your burner and the other end connects to your incoming gas line. This fitting is designed to draw in air to mix with the LP. This is done so the LP gas flame will be cleaner and brighter.
Solid Brass Adjustable Air Intake 90,000 BTU Orifice Model Has 1/2" Female NPT and 3/8" Male Flared 150,000 BTU Orifice Model Has 1/2" Male NPT and 1/2" Female NPT Coupler 300,000 BTU Orifice Model Has 3/4" Male NPT and 3/4" Female NPT Coupler