Any exhaust leak you can hear that is up close to the engine[before the exhaust pipe goes 2 or 3 ft.] will suck raw air in and cause the backfire. I have seen people use a product called [muffler mend] or something like that to stop the raw air from getting mixed with unburnt fuel. This [muffler mend] comes in a tube like calking. You put it on when the exhaust is cold and then start up the engine and let idle till the exhaust is hot then shut it off and let it set for about 30 minutes. This will let it set up like cement and it should stop the air leak. I just thought , I believe it is called muffler cement.

kdot em3