I have a 99 310 with MIE 350 MAG...

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I have a 99 310 with MIE 350 MAG MPI V DRIVES PORT ENGINE HAS LOSS OF POWER WON'T RUN OVER 3,000 RPM AT FULL THROTTLE 

[wingless]

Does the problem still exist when the keyboard capital letters are not locked on?

Assuming the starboard engine gets up to full rpm, such as 4,600 rpm, is that correct?

If both engines are affected then the first thing to inspect / resolve is marine growth on the hull and the running gear. That is the most common issue for the symptom.

Afterwards, then all the standard engine tests are required, including: fuel pressure; fuel pressure versus rpm; timing; spark plug gap; compression; flame arrestor cleaning and intake plenum cleaning.

Many of these require skills, tools and factory documentation, so don't jump in w/o being prepared.

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It is port engine only boat ran good when bought it in March I live in Colorado boat in Florida noticed problem when there in October have a mechanic working on it full tune up done fuel pressure next to check what about an injector problem boat is in dry storage

[wingless]

On my 454 MAG MPI engines at one point I suffered decreased performance, going through all the previously listed steps, finding / correcting some issues and confirming acceptable on the others.

All sixteen injectors were all sent out to a company for before / after testing and cleaning. Several were incapable of restoration, requiring replacement.

The removal and cleaning of the intake plenum is part of my annual maintenance. It is required on my engine to maintain normal performance.

Both the fuel pressure and the fuel pressure regulator should be checked per the Factory Service Manual (FSM).

Other normal stuff, like distributor cap / rotor inspection / replacement and spark plug wire testing / replacement are part of the effort to restore normal performance.

Assuming salt water operation now, w/ raw water cooled exhaust, don't forget to include annual exhaust disassembly for internal raw water passage inspection and periodic exhaust replacement. The typical cycle is 3-5 years for OEM parts.

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Found no compression # 4 cylinder going to pull head next

[wingless]

That is very unfortunate.

One common effect from neglected raw water cooled exhaust is severe or total engine damage, including valve failure. That is why annual disassembly for inspection / periodic replacement is recommended.

Hopefully the engine restoration is possible at an acceptable cost.

[Bobcat]

Sounds like you need cylinder heads and manifold and risers like I had to do

[PHM]

Is this also true for freshwater cooled exhaust, just takes longer? What is involved in inspection? I get lots of corrosion flakes when draining to winterize. Great Lakes Cruiser

[Genny]

Don't want to sound negative, but I experienced the exact symptoms in my '01 310......
Sorry to report #4 cyl dead (0 cmprshn), 50% cmprshn in #5 & 6

[Genny]

Still waiting for 2 new 383 strokers, from Merc...(ordered July, 2020)
Has anyone else experienced waiting th as long for new engines, or parts??