Re: Preplexing Engine Issue
Oil in the magneto cover could be a result of a crankshaft seal going bad. This can cause some running issues as it allows air to enter and make for a lean running condition. It can also compromise the magneto if the oil is sufficient to shunt across the leads.
It sounds like you have a good handle on troubleshooting so I can only armchair guess at the problem. If the plug is dark but not wet then that indicates a slight rich condition but with so many miles on your engine I hesitate to say that is the issue here.
How does it idle? If the idle speed is elevated then that too can represent an air leak.
I doubt seriously that we're looking at a worn out engine. There have been many that have accrued lots more miles than what you have without trouble.
Is there any signs of oil/gas around the head gasket or cylinder to case gasket?
How about a partially plugged exhaust. The muffler, especially the catalytic converter style, have been known to get plugged/fouled with oil and exhaust residue. A partially plugged exhaust is another thing that can cause a decrease in performance. I hope we can solve this issue for you soon. Maybe a few more guys will chime in with their opinions too. Guys???
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