This is an interesting old thread I just stumbled upon. I get backfiring when it is colder and drier. I figured I was just running a little lean in the cold air since I also chug a little at idle with 3 turns on the screws. Was going to go back out .5 turns on the screws this weekend to see if this helped.
FYI - start it up in the garage for maximum effect on the neighborhood.
