If the dealer is no help..... The Vols backfire becuase of the pair valve. The pair valve is designed to help burn unburnt fuel in the exhust system, when they screw up the bike will backfire. They are for emission control and disabling them will not effect the performance of the bike. The pair valve is on the left side under the chrome oval cover. There are 3 hoses on the back of the valve 2 connect to chrome tubes. Disconnect these 2 hoses from the back of the pair valve, stuff a golf tee into each one. Put the head end in first and leave some of the tee hanging out of the hose in case you want to remove the tees later. Reconnect the blocked hoses to the valve and reassemble. If it doesn't help just pull it apart and remove the tees I have never seen it not help. Takes about 15mins to find out.
I have not backfired in 9k miles....
This guy sells a kit to completly remove the valve and go "naked jugs".
http://www.moccsplace.com/images/pair/pair.htm