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It is unlikely that two different carbs would both cause lean misfiring and backfiring through the carb. Rule out other fundamental sources, like valve adjustment, timing and stuck centrifugal advance.
Well, your idle jet suggests that the carb isn't necessarily jetted properly for your 2F at Pennsylvania altitude.
So a 60 Idle jet sounds about right for 194 foot above sea level...Jets get smaller at higher altitude, but, according to the table (not an O2 sensor) a 45 would be for my altitude or above. If nothing else, it suggests looking further. However, I'd have an air horn / bowl gasket ready in the event that it is stuck in place.
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Plugs are carbon fouled. Could be normal w/ new engine break in and burning off assembly lube. Install new set of W14EXR-U or BPR4EY (hottest numbers available) and see how they look......
...........Focus on the important stuff: getting the cold BPR5's replaced with the correct BPR4s............
Then drive it hard for a while & re-check plugs.
Recently purchased my 1976 2F. It came with a 32/36 and ran poorly. Similar symptoms to yours. I cleaned and rebuilt the carb with marginal improvement. Plugs improved it a little more. Timing solved the problem to 95% perfect. Not on the BB but a little advanced when timing with vacuum disconnected and plugged. Turning the distributor (HEI) about 4* counter clockwise seemed to be the sweet spot. Don’t know why, but it solved the backfire & mid throttle stumble. It still doesn’t like me to totally mash the pedal, but a good moderate throttle press is tolerated nicely and the rig accelerates away.