[ophic]

Mine had a huge backfire around this rev point on overrun. Found it to be that the garage had left a PAIR tube unattached. I was normally on the bike so couldn't tell if it was chucking flames but it sounded like a shotgun going off and generally made me jump out of my seat.

So... check PAIR tubes.

After that, I'd say check spark plugs. If unburnt fuel is getting to the exhaust, it means it's not being burnt in the cylinder so could be a weak spark or dirty plug.