My Chevy 350 jet boat fouls plugs #3 and #6. I noticed that the Holley carb vacuum port for the PCV feeds into the dual plane rpm manifold adjacent to the fouling cylinders. The plugs are darker and black up the threads but not shiney or gooey. The engine runs at 4,400 rpm's for sometimes an hour at a time. It has consumed some oil, maybe 1 quart per 10 running hours. I have Profitm valve covers with factory style baffles, a generic GM PCV and a push-in breather on the other cover. Is this set up likely causing the fouling on the third plug back on each side? The other plugs are perfect, new rebuild.