Following a suggestion from a very reliable source, I pulled the heads. Three of the four combustion chambers were as clean as a whistle. There was no carbon buildup. So I purchased some new head gaskets. When I laid one of the old gaskets over one of the new gaskets I discovered I was sold the wrong head gaskets 10 years ago. The gaskets I took out were for a larger bore engine. I now have the proper head gaskets installed.
It only took me about 5 years to determine the problem. Sometimes I'm a little slow.
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