I recently did a compression test on my 1984 Mercury 50hp. All 4 cyl are at 125 and that's good but while cranking the engine I noticed small flakes of black carbon coming out of the spark plug holes. I shined a light inside and all 4 appear to have thick carbon on the piston heads. How do you clean them? I know my dad use to say if you slowly poured a mix of transmission fluid and water in the carb or your car while it was running it would clean the carbon off the pistons.