Backfiring is a result of a too lean condition in the low throttle range which is controlled by the pilot jet circuit.  By removing the brass plug so you can richen the pilot circuit mixture is the way to stop/reduce the backfiring.  If backing out the pilot air adjusting screw to "3 turns out", if necessary,  is not enough to stop the backfiring then you will need to go one size larger on the pilot jet...then readjust the screw.