Ken,
Do you see gas leaking at the throttle pivot (or smell gas while running, or after shutting off the engine)?
I still have a similar backfire problem with my 261 with dual Rochester carbs. After doing most of the things mentioned above, the backfiring decreased but is still there when I back off (allow the engine to brake the truck).
My guess is that I have a bad float adjustment in one of my carbs (as suggested by Trapperschevy, above). After I shut off the engine, I can see a small amount of gas leaking out of one carb (my carbs have new bushings on the throttle pivot, so that is not the cause of the gas leaking). I have been too lazy to take it apart and check the float adjustment. If the float is set wrong, I think that gas can continuously dribble into the intake manifold (I have been told that this would result in excellent conditions for backfiring).
Good luck,
Tim
