What brand of fuel is it? These might be the dreaded "taters" that Brett has mentioned several times in the past. he has seen them accumulate when using SIG fuel. I use SIG fuel and have never experienced them, but I don't run piped engines. When this has been discussed, I have wondered if this was a phenomenon related to using a tuned pipe. But I doubt that you are flying a piped engine, knowing you from your posts over the years. Gotta be something in the fuel or being ingested by the engine when running.
Type at you later,
Dan McEntee