Having this problem for about a week decided ill finally get an opinon on it.
Holley 750 on a crate 383 motor with a tko500 trans. Whats happening is when regular driving I take off shift thru a few gears then when slowing down for lights, stop signs etc, I get multiple major backfires until the car is back in neutral. I also usually get a backfire shifting up from gear to gear at higher rpms. Ive already checked the timing, plugs, wires, cap, etc. Im thinking perhaps it has something to do with the fuel delivery, probably too much - running about 6psi. Only other thought is I switched to a hotter plug at 43 to a 44. Not sure if it would make that much of a difference though.
Any thoughts??

 

Bump

 

When I had an exhaust leak my car would backfire a lot... I doubt this is your problem, but just throwing it out there.

It was doing it between shifts (I've got a manual also) but I only noticed it on upshifts. For downshifts I generally just throw it in neutral and let it roll. Either way, the backfiring was really throwing me off and I had no idea waht the problem was. It would generally do it whenever I revved the engine and let it come back down, so as I got off the gas for a shift, the revs would drop and I'd get some backfires, sounds somewhat similar to your problem to me. Anyway, I was changing the oil a couple of days later and noticed that about half of my collector bolts on my long tubes had worked themselves out and headed for greener pastures so I had some huge exhaust leaks. So I figure hot unburnt fuel was being introduced to oxygen midway through the exhaust system and managing to combust.