Pinging

Water or trans fluid dripped down the carb while reving it 2500+ should knock the carbon off the pistons, not to much fluid though and be prepared for lots of white smoke. However This can some times cause larger pieces of carbon to get stuck on the valve seat burning it, but rarely.
If it pings that bad you can bet octane alone will not fix it, the carbon is glowing like a 'glow plug' igniting the fuel before the spark.