Mr. Fixit -
No, I'm not sure. I pulled the intake manifold and head on the side with the problem. It appeared that the intake manifold gasket was allowing water to leak into the intake runner. There was no obvious sign that
oil was also leaking past the bad gasket, but it's possible. The head gasket looked ok. I'm in the process of putting it back together now. ... Having found no obvious explanation for the rapid carbon buildup on plugs 1-4, I am looking for other possible causes in case the phenomenon continues after the gaskets are replaced. .. I appreciate you taking time to help.