Hi there. I have also got a 2007 Boulevard s40 and I have read on a few forums that the backfiring is a problem that is associated with not only the Boulevard but other bikes as well. Someone informed me that it was something to do with e required modification by the EPA that gets rid of unburnt fule, or something like that. Apart from being an annoying loud noise I don’t think it relates to anything being actually wrong with the bike.
How do you like the bike other than that though? I love mine and I am so happy that I managed to trade up to it.