The Stovebolt Site Forums The Shop Area Engine Shop tried everything - still a terrible miss

I would check valve clearance first.

Then check for vacuum leaks, propane from an unlit torch around the manifold/head connections or squirt them with oil.

Black plugs could be caused by a low vacuum condition causing the carb power circuit to open.

Put a vacuum gage on it and check the level and pulsing if any.

Let us know.