what kind of motor? two stroke outboards get some carbon buildup after many hours of lower rpm-1500-2500 or so. its not harmful to the motor if you burn it back off.there's a decarb method to get it off the pistons that works well. google dunks decarb. it will help.
A 2 stroke premix or oil injected motor doesn't like to idle all day. On the way back to the dock run it hard and that will help burn off some of the carbonn build up from excessive idling. We also decarbonize motors.