Hard to say. Almost sounds like it's floating a valve, but I reall need to see it. Traditionally, a backfire like that happens the split second after you release the throttle, or start to, and is usually caused by a hole or an opening along the pipes (usually where the pipes bolt to the headers). What happens is: when you are in the throttle, and suddenly back off, even just a little bit, you create a vacuum. This applies to the exhaust, too. The vacuum pulls oxygen rich air into a very hot spot in the exhaust, and it ignites.
If that's not your case, then you probably don't have enough back pressure.
Take care
~Chris
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