Seems like we've got enough data already to figure this out.
It's not ignition since the problem didn't migrate swapping plug leads.
The mixture screw doesn't change idle speed.
Popping in the exhaust.
That tells me the right side is running rich. For whatever reason.
Maybe the butterfly is loose and way off center.
I was thinking you might have some compromised o-rings, but seems that would make for a lean mixture, not rich. Still, I'd check on that. Those o-rings can get split on the sharp edge of the o-ring groove and it'll be hidden from view. Gotta look close and prod a little to see it.
The choke is the other possibility. Not too many places where EXTRA gas will come from.
As difficult as it's been to track this one down, I'd say it will require some close scrutiny to figure out.
Wanted: Dead, smashed, crashed or trashed gauges
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