Engine slows down

so, a quick question for some of you mechanically inclined, with experience with this year/style engine.
2002-340. 6.2 mx engines.
Stbd engine was said to be freshly rebuilt when i bought. Both orginal 650 hours.
Both start idle perfectly. Run super smooth throughout rpm range.
However, full throttle, after warm up, port engine 5k rpm ish.
Stbd engine may hit 5k briefy but then gradually slows down to 3500.
No butps, farts, sneezes, just looses power. Havent changed anything except fuel/water seperator/ oil/ basic maintence.
This is my first mpi. Have service manual. Where to look first?
Thanks

A Fuel pressure test would be tops on my list.

Set up a gauge and have a helper record the fuel pressure as you drive.

next time it happens open the fuel filler cap for the starboard fuel tank...if the problem goes away the vent line for the starboard fuel tank is clogged.....

cliff

Thats a distinct possibility. That tank you must put fuel in slowly or it will burp it back at you.
The port doesnt have that problem at all.

I think you found your problem then... Same thing happened to a guy on my dock, once the vent line was blown clear he was good to go.

This reminds me of a time we were up in Pentwater a couple of years ago, and a guy and his wife were slipped next to us and had a similar problem with their Regal. All of us dock mechanics narrowed the problem down to his starboard vent line, and he said he could see some type of spider nest in there by leaning overboard and looking in there with a flashlight. We all suggested he disconnect the hose from the inside and blow or suck it out. Next thing we know he's leaning overboard with the central vac in hand. I'm pretty sure you all know what happened from there. He got the nest out all right, and filled his vac with gas. They were still there when we left, and still trying to air that boat out. I believe they rented a place in town to stay while they worked things out.

-Tom