Haha! Don't worry about the loud bang, I've had a few - its just some petrol vapour igniting in the exhaust and not in the engine - perhaps some unburnt fuel made it through to the exhaust or something when you turned it off the time before the bang.
A carb balance will help with the miss-fires when the bike is running; i assume its due to being ever so slightly "rich". DO you have an aftermarket loud exhaust fitted? they can induce a bit of popping and banging
