To be honest I'm not sure what causes the backfire when starting but I still get it myself ocasionally.
As far as your concerned, if you have put the tors on I'd be changing my main jets to 125's or thereabouts and getting the thumb nail screws most of us get so you can simply adjust the air fuel mixture.
Once you've done that, you can look at getting it sorted so the loud bang should disappear.
That's my experience anyway, but hopefully others with your setup may chime in here to add from what I've said cos mine wasn't stock for very long.
