Moggy.
If you mean backfiring like in the cartoons, sounds like a shotgun and smoke out of the exhaust then I'd be concerned.
If you've set off straight after starting and there's a little pooping going on I wouldn't worry too much, it's probably slightly generous fuelling by the ecu.
I can't say if all 650's of your model do it, for one good reason, I haven't got a clue. But if it runs and rides OK once warm, I'd ride.
Upt'North.