Hello Dan,
Assuming the appliance is specified for Propane, and that the gas is supplied at the correct pressure (Regulator) then sooting strongly points to to a problem with mixing the gas and air in the venturi.
This is likely to be blockage (insect)of dirt in or around the injector.
Cleaning the venturi and burner in warm soapy water and using a nylon bottle brush may help Do not use a wire brush as this may damage the injector assembly.
Rinse and dry the parts (in an electric fan oven at low setting about 75C works well)
If any any doubt refer the cleaning to a gas fitter.
If you are and drying the burn