I recall a wealthy customer of a Cadillac salesman, Johnny C., coming in to the dealership, her caddie was coughing and spitting and running very poorly. Someone in service got to her before Johnny showed up that morning, said her motor was kaput and it would need a major overhaul. Johnny came in, found out what was going on, told the lady to take a seat. He knew that all she did, every day, was drive a few miles slowly in and around Highland Park. He took the keys from the service writer and in the next moment it seemed, there he went screaming down Preston Rd. About 20 minutes later, he pulled back in, walked over to the lady and said she was ready to go, no charge, and that he'd see her later.
We asked him what the heck had he done? Johnny said that he knew the old lady very likely had a build up of carbon from low speed and infrequent driving, that there was nothing truly wrong with the engine. His old-time remedy? He said he added a few gallons of diesel fuel to the tank, he either advanced or retarded the timing a few degrees (I can't recall which), then took the car down the road at full throttle in its first gear automatic transmission setting, got to the highway, shifted to drive, and opened it all the way up racing down a few miles and back on North Central Expressway. By the time he returned to the dealership, all of the carbon had been dislodged. The car ran perfectly smooth, not a hitch.
He saved the lady from an "over diagnosis" and an expensive but needless repair job.
***I have no idea whether any of this would work on a boat, but it just goes to show how carbon can contribute to a loss of performance. There are some nice products out there, today, to control carbon build up. No need to lubricate an engine with a few gallons of diesel . . . but it sure worked 40 years ago!