If the presets are to rich you could carbon up the rings, to lean and you could burn down the engine.
Adjust the carb so it is a tiny bit rich (100 RPM rich at WOT), as the engine breaks in you will need to readjust.
It ain't rocket science. Proper tuning will make YOU and your engine happier.
Black spots... too much oil, bad oil, too rich, engine is still breaking in...