I can't help thinking this has origins in oil leaks. There are all sorts of seams in these engines though which oil can ooze out. Among other things it could get on the exhaust risers and cooked. One thing I routinely check is the screws that hold the valve covers on the cylinder heads. Those things are annoyingly prone to coming loose and letting oil out of the engine.
I don't have a ready explanation for the rain factor. A nascent theory is that in heavy rain water will be coming into the cowling, then into the plenum above the engine and getting who knows where on the engine proper. Given the temperatures in there some would be boiling off. But some likely remains in a liquid state to carry various deposits with it as it runs out.
Soot on fuselage
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Re: Soot on fuselage
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Re: Soot on fuselage
Rain? In California? Charles is right here in the SF Bay Area. Where did you find rain? Did you take a trip to the midwest or east?