It sounds ok, but a bit weak rolling over.
Like he said, jump it off a car (not running), just make sure you're getting a good connection. A running car alternator could possibly fry the R/R, but you can hook a 1000 amp bulldozer battery to it and it won't hurt a thing.
It didn't fire once.
Plugs wet or dry after that?
Also as said, backfire is usually a sign of low voltage on this bike. Don't recall any other machine being like that, but this one is for sure.