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Will, to clarify your carb adjustments, assuming you have an original Marvel-Schebler carb, the big adjust screw is the "power" mixture. That should be set initially at ~1 3/8 turns out. That screw leans your mix when turned in. The smaller screw is your idle mix set screw. It should be turned ~7/8 turns open. That screw richens the mix when turned-in. The idle mix screw only affects mixture at idle, say ~500-600 RPM. Above that speed, your big "power" screw controls your mixture. These carbs are intended to run a bit rich at full throttle, it is one of the features to keep your engine from overheating under heavy load. Adjust your carb as described above, then see how she runs. You may need to adjust the idle speed stop screw on the back also to adjust your idle speed once you get the idle mix right.
Once your mixtures are set, ramp-up your engine, and also try letting off the clutch rather fast, if the engine hesitates when you do that, richen your power screw a little until it stops hesitating. I highly recommend you also check your spark, weak ignition is a common cause for sooty plugs. You should have a nice zappy blue spark, not a weak orange spark. Dirty connections, and improper point gap are the 2 biggest contributors to weak spark Good luck !
George
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