I find, with St G20-15's, the best runs are when the exhaust is clear. A little honey-colored is OK. Also as an engine is run more, I find that the compression needs to be very gradually backed off, exhaust clears, and performance improves. If you are running dark brown to black, I think you are overcompressed. Also, I think you are running more prop than your engine likes. I would try a 9 x 6. Diesels are, I think, more idiosyncratic than glows, and what is true of one may not be true of another.