OK on your set up so far. Since you obviously have the carb off, it might be worthwhile to swap it with another, and see if this one particular problem goes away or not. Then you see if you should continue on the carb path, or look elsewhere.
When you say "backfire through the carb", and "seems to be getting worse", that's what gets my attention.
Good luck, hope you find the (simple) problem soon.
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