I leave the carbon coating alone, only if it starts to build up a crust layer on the top, then maybe remove it with polish, but never touch the sides. The crust layer will actually start to raise the compression over time. I also look at the sides of the piston at the carbon coating. This give you a good look a how well the piston is sealing to the sleeve. You can also clean the head button and run the engine and look at that for mixture tune, but like someone else said, its the fuel you run that will have different colors or hues, so you have to know you're fuel to do this.