I agree with what others have said here, it does sound like you have a leak, compression loss is my best guess, I had a crank seal behind Mag cover fail once on an engine, it was like having cruise control, bike would cruise at 25mph without even touching throttle, major lean condition, crank bearing seized and spun in case which ate away at case until mag flywheel chewed into seal and caused the serious leakage.........
( a small rant at myself here)....!Was a dang good and smooth running Slant BGF engine too, still need to break down and buy a new case to put all those good guts back in and I'll have another good one on the shelf if I should need it......wish I would have been using the Opti2 in this one, may or may not have made a difference, I guess I'll never know, just know I've not had a single failure since that one and I've been using Opti2 religiously since.
Best wishes, hope engine is OK and it's a simple torque it back down or just a gasket replacement issue, if you need a gasket ThatsDax is a good source at only $1.99 for a new head gasket.
One more thing that just came to mind, carb could have some trash in it that has the main jet partially stopped up, if you find that all gaskets are sealed properly you may pull carb and give it a good spraying out after removing fuel bowl to inspect for junk that may have made into it.
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