Blower Stuff-Up . . . Solutions ?

[Capt Phil]

On a recent trip back from the Channel Islands to San Diego, the port engine on a 65 foot powerboat with Detroit 8V71's began overheating at anything over about 1350 RPM. Checked the raw water strainer, all hose connections, etc. Heat exchanger had been rebuilt less than 500 hours before.
I decided to run the starboard engine around 1500 RPM but backed off on the port engine to about 1250 RPM that kept the temp gauge below 195 F. As we entered San Diego, the port engine began to blow black smoke, a sure sign the blower was stuffed up with unburnt fuel.
Now looking at a big repair bill plus an awful mess to clean up on the transom and stern windows.
Have not yet checked out the water pump">raw water pump on the engine which may be the culprit, however, I doubt it.
I feel a bit of an idiot for not scoping the overheating problem out sooner when we were anchored.
The boat had not been used for several months but ran fine on a few turns around the harbor to check out the systems for a couple of hours. Also, she performed well on her trip to the Islands, about 85 NM's one way.
I guess the reason I'm posting this is to let others learn from my mistake of not running the engines up to proper operating temp under load and watching for oveheating signs for a considerable period of time. Capt Phil