You say "battery is charged", but have you inspected the connections? Just covering all the bases. Make sure they are fully contacting and clean, this bike likes a lot of current when starting. Check this and piston height.
That "bang" as a bad sign though. My red bike had it happen to it. I got about 25k out of it before the #3 rod bearing went (which is a trait of this motor). If you have a leaky petcock and weak float needles, gas will get by them. If you keep the bike on the centerstand gas WILL flow into the carbs , then engine, and raise the oil level (check this too). If you keep it on the sidestand, gas will most of the time pour onto the floor from the airbox (and maybe burn the house down).