Hello George:
I had the same indications. Pulled the engine, and completely
rebuilt it anyway. Invited an old salt (Al D.) over to my boat, and he
interviewed me over a sixpack. He took another swig on his beer, and
said "intake manifold". Tryin to catch up I took 2 more swigs, and
said No Way. We eventually ran out of beer and I removed the Carb, and
took a flashlight to the intake ports. One was stepped up, and
couldn't tell if Al was right or not. After removing the intake
manifold I rolled it over to find a tin plate on the bottom. I popped
the rivets holding the tin and lo and behold there was a hole about
the size of 2 golf balls in the cast iron. It seems that if you do not
get the engine hot enough to evaporate the moisture there it will
definately rust it away. I found the same situation in the port
engine. This brings back fond memories of San Diego, and Al.
I aint no "old salt" but I hope this expands your thinking.

Den 48ftYF EAGLE
http://www.densnet.com