Sounds like timing and fuel mapping needs to be fixed, Id get someone in the boat that can tune that MEFI..........where are you located

NOT to sound harsh but lets ask what dealer - a marine certified dealer and of what brands ?? why would whatever dealer out of the blue just come up with the boat needs bigger injectors ?? IMO they sound like they know nothing about marine engine systems unless what was the reasons for bigger injectors and they have have that correct answer.

If I were you I would find out what exactly you have for an engine and set up - then find a real marine shop that can get you up and running correctly.

Posting up here without eyes - ears - hands and proper diagnostics tools is not the way to go about fixing your app. Total waste of time and money for you inwhich is throwing darts at a unknown dart board.for everyone.

. Your engine possible might have a totally mix match set up with totally wrong and systems installed that might never run correctly.. So what is stock parts on your app and what is not stock on your engine ? What ecm do you have ? Where did you buy this boat ? who did the engine - the past owner of the boat ? anyways good luck

You'll need all the part #s(broken record here) and then call,or better yet...visit a place that reprograms the MEFI ECU's.

All EFI engines (cars, boats, sleds, motorcycles,etc) ,when things are changed (especially cams/inectors.etc) needs to go thru this.

There are a few good programmers avail by phone but that is 2nd to having someone be able to do it in person. Hopefully someone here knows of one in our state.

borrow a intake manifold&carb and install it,you just might be pleasently suprised.

May sound crazy but have you verified your exhaust is good? Had an issue with the 454 in my Powerquest - misfired under light load in midrange rpm and seemed to clear up when wide open. Turned out midrange operation produced enough pressure for leaks that killed a couple cylinders, wide open it wasn’t really noticeable other than losing 200 rpm on top end. Would be easy to rule out in your case by looking at plugs and pulling risers and looking.