Steve,
My guess is that a lot of things might cause the symptoms you describe.
Last year, I had very similar symptoms and figured the "backfiring" through the exhaust pipe was a result of unburnt gas in the exhaust system (perhaps assisted by air leaking into the exhaust system).
My problem was solved by resetting the level of a carburetor float - the float was rising too high and gas was dripping into the intake at all times.
Maybe others will have more helpful suggestions.
Good luck,
Tim
