Clyde, I recently had a back-fire problem and it turned out to be an exhaust leak. The flange gasket between the manifold and exhaust pipe was split. Headers are notorious for leaking, too.
The exhaust gas that exits from the cylinders is hot enough to burn, but with no oxygen present, it will not. However, if any outside air can get into the pipe, especially on deceleration and a rich mixture then present, the exhaust will ignite......hince your backfire. Check it out! Dennis
