The only orifices I've seen on these old mobile home single burner units are located on the regulated side of the gas valve, so that the orifice inserts into the hole in the spud plate, see pic. However, I know that you know where the orifice goes:biggrin2:Usually it is in the burner orifice.
FYI a partly plugged intake can also starve the burner(s) for air and cause soot.
Old school burners used to have adjustable Primary air shutters on the intake and if the burner tube got dirty from years of sucking in dirty air then it can burn dirty.
If those shutters were set too much closed you get a dirty flame. Conversion burners also have them and you may see them on upshot burners for mobile homes.
Newer burners have diffuser rings at the end of them to mix the gas and air and even they can eventually get dirty.