When I pulled my engine, my plugs were in that exact same condition from the previous owner and the truck was running like crap. Spitting black tar out the back and smelled really rich! I had a blown head gasket and a bad header gasket. This was the condition I bought the truck in so figured just change gaskets. When I pulled my intake, it was completely clogged with solid carbon. There was only about a pencil eraser size hole on the largest port on one side! The heads had the springs too tight and no seals on them either. I'm not sure if this is what caused the build up, but after my top end rebuild they stay clean. I cleaned up the intake, re-used it, and have not had any issues since. I'm not a mechanic but that what my issue was. Maybe you just needed new plugs?
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