Misfire at cruise, popping/afterfire at 3000RPM

Righty Tighty said

@George Jets I'm definitely gonna pull the filter and cut it open, but I’m just going to plan on a rebuild at this point.

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Though not what you wanted to hear, it is good because now you can get it built the way you want and will know "how" it was built. Lots of good stuff is available for our engines that was not 20+ years ago!