I need a better picture in natural light.
If the "carbon" wipes off with a rag, the top of the piston is not experiencing enough heat to form what I call carbon deposits. The cold areas of the piston crowns will not form hard-dry carbon deposits that must be scraped off with a blade or wire brush. Hard-dry carbon will only form on the areas on top of the piston that is very hot.
A well tuned carburetor on a highly developed engine will not ever see high enough piston crown temperatures to form hard-dry carbon if the engine is not ridden hard. Running the engine through 2 or three gears at wide open throttle and then putting around for a minute or two before giving the engine another WOT short blast, will usually pile up a black gooey film on top of the piston and inside the exhaust pipe and muffler. The jetting, brand of oil and or the fuel to oil ratio will not have much influence on eliminating this gooey film if this is your style of riding.
If the jetting is severely rich, this dark goo will not form because the engine will have a lot of rich misfiring and the majority of the piston crown will be too "cold" to form the black goo.
