Originally posted by chachi
What is causing a layer of black/dark-brown deposit to form on my piston and button? Is it from running stale or bad fuel? Or is it from running my engine out of tune? The only substance that seems capable of removing the deposit is gentle polish. Engine cleaner won't.
It's carbon deposits from the oil than burns off the fuel during combustion. Depends on fuel you are using, but mainly because of over rich mixture. Now, all fuels leave that layer over a long period of running, that's why I clean them after every 3 races, but if you're getting those layers after one run, your engine is tuned to rich.