ORIGINAL: spaceworm
It sounds like a small piece of dirt or maybe carbon got caught momentarily in the piston skirt between it and the cylinder wall. I would not be concerned as it is, but I would watch to see if it gets bigger or longer. Do you have much carbon in the muffler used just before the occurrance? Do you filter your fuel? Do you use an air filter if you fly where dust or dirt debris could be sucked in the carb intake? Good luck, I hope it does not present itself as a real problem.
Sincerely,
Richard
The muffler used extensively on this engine had no accumulated carbon and the fuel is filtered several times via felt clinks before it reaches the carb, and even though I fly from grass fields I have never used an air filter on any of my gas engines. Maybe it's time to think about using one on this engine as it is on a Stick model which is uncowled. Hopefully someone will point me to a possible filter source or suggest a method of adding a filter.
Karol