Probably normal for lawnboy oil. But not good for the engine.

Are you POSITIVE that you have it tuned and leaned out on the top end? Running excessively rich will cause carbon buildup.

I'd switch oil now. The popular thing to do is to switch to a synthetic now. I don't use synthetics and I don't have carbon problems. But it's up to you. It's your engine. I'd switch to a high quality oil and lean it out to 40:1 or 50:1. Then TUNE the needles again. Lean the high speed needle to max RPM. Use a tach if you have too. When you hit peak RPMs, richen it up about 200-300RPM and go fly it.

I'd also clean that carbon off the top of the piston before you move on. You already have it apart, might as well get rid of the carbon. I've never had carbon like that, so somebody else will have to tell you how to scrape it off. DO NOT use a steel brush!! I do know that to be true.