[2.2 Straight six]

do you have silencers cats an stuff after ?

the cause of backfiring, when talking about the "bang" an flame out'a the exhaust is unburnt fuel travelling down the exhaust system an ignighting when it meets the air.

see, when the throttle's open there's enough air to burn the fuel. when you suddenly lift off the fuel goes into the chamber, but there isnt enough air for it to burn properly, so it doesnt. then on the exhaust stroke it goes down the exhaust, while extremely hot, but still unburnt. then it meets the air as it exits the exhaust and ignites.

you dont often get this with unmodified engines because its all cooled or whatever before it meets the tail pipe, if you have open pipes that the thing above is why it backfires.