I am not positive on this one, but with that little back pressure making the head perform a lot better, some of the unburnt gasses are finishing in the exhaust system and causing the popping noise you are hearing. I think that is normal usually, but with all the normal cat. and muffler system, it gets muffled out. You will notice most NASCAR cars doing this while letting off the gas in the turns. They don't get the best fuel milage, but for the power they produce, they do run as lean as they can and have that nice flame due to the overlapping of the cam intake and exhaust openings and not as much air due to the carb being closed.
Just a thought though.
Russ
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