Gasket Set-Engine

Hello mwrought Try adjusting the valves. I am thinking the plug smudge may be from either carbon, or a rich fuel mixture or possibly both. I have seen plug smudge before with a valve out of adjustment.
I also have seen an engine with a partially sheared flywheel key; a few degrees out of time produce a carboned plug. The flywheel key may need to be checked as well.

I have a question about removing the flywheel bolt. I've tugged on the top of my Briggs 17.5 Intek and don't want to snap anything (engine or me). Is the top bolt left hand thread or should I put more grunt into it?

Hello, You are going to need to make sure the engine cannot turn over if you are attempting to remove the flywheel Bolt. Since you stated the engine runs fine, I feel that the engine may just be getting too rich of a fuel mixture. Carbon is a byproduct of incomplete combustion in an engine. Carbon on the plug tells us that is what is probably happening. The engine is failing to burn all the fuel going into it, possibly.
Maybe you would like to remove and clean the carburetor. That is a lot easier than removing the flywheel on the 17.5HP OHV Briggs. I would tune up the fuel system before removing the flywheel.
(By the way, I also have a 17.5HP Briggs engine on a lawn tractor.)
A larger engine like ours that is out of time with a sheared or partially sheared flywheel key will run poorly (not fine), produce black smoke and smudge the plug.
Have you HIT an immovable object while running your engine? That is what usually shears the key; the engine coming to a sudden stop when running at WOT.