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Sounds like a lean condition, it WOULD be for sure if you were running Webers. It's a little bit of work, but swap the jets from one car to the other and see if the backfiring moves with it. If yes, it's in the carb(s), if no, it's a tight valve or air leak . . . I think . . . worth a try. Whenever something drives me crazy, it usually take a crazy solution to fix it.
Luck,
TC
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