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Sounds like an oil fouling problem to me. It could be a loose valve guide or damaged piston ring or cylinder wall. Airplane guys have a scope that pokes through the spark plug hole and allows you to see the cylinder wall but its easy to remove the head if you have a gasket. If its a ring it will show on the wall. Try wiggling the valves at the top end to find a loose valve guide. Mielage info would help this diagnosis. Higher meilage engines have more valve problems. The last time I saw this problem the oil control ring had a problem and one new valve guide was larger than spec ID. The two worked together to use oil and foul the plug.
Capt Frank
Capt Frank
