I would bet that you have a vacuum leak on the front cylinder. Maybe a cracked boot. Maybe you didn't get the carb all the way into the boot. Maybe you have a vacuum line that is loose. I had this for many days and just couldn't figure out what was wrong, and then I found that one of the vacuum plugs I'd put on was blown off. That causes a super lean condition on one cylinder, and it will backfire into that one carb all day long. I'd put money on a vacuum leak.
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