Hi
Maybe this has already been said (not taken the time to read all the replys sorry). The bang I guess is unburnt fuel detonating i guess indicating a problem with the ignition circuit.
I would be supprised if it was a loose connection if this was not occurring before you layed the bike up. unless of course you have disturbed something during this time while cleaning or maintaining the bike.
I agree with the reply suggesting the fuel quality but you have refilled with fresh (is their an inline fuel filter?). I guess although it backfires it does start if so is it missfiring or running only on one cyclinder?
Check the obvious plugs, leads, crimped fuel hoses, air filter is not blocked and damp within connectors (WD40 will sort this) etc check one thing at a time and if its still a problem move on to the next.
At least do this before you consider asking your dealer to look at it.
Regards
DaveO
Threaded Mode