Originally Posted by fyrstormer

For anyone still keeping score at home: After thoroughly degreasing and re-oiling the internal pullstart bearing, the strange black goop turned into strange brown goop, but there was still strange goop accumulating on the top of the piston regardless of its color. Later, I increased the compression ratio by dropping the shim stack from the factory-set 0.55mm to 0.45mm, and the strange brown goop disappeared within a couple runs. I guess the higher compression ratio is causing the piston to get a little hotter, so the half-burned oil vapor left-over from combustion is no longer condensing on the piston. Mystery solved.

The engine continues to run excellently, except for an irritating tendency to instantly stall when revving-up after idling for a minute or so, but that problem was also apparent with the previous engine from a different manufacturer, so it can't be a problem with the engine itself. I'll figure it out someday.