5.0 MPI Bravo 3 (2005) Mercruiser won't accelerate to full throttle

[Maiden Offer]

I have a 2005 Searay 240 Sundeck with a 5.0 MPI Bravo 3 I/O engine. It has a Cool fuel box. It had been running fine and at full acceleration would top out at approximately 48 to 50 MPH. Suddenly during use it slowed down and now tops out at about 32 MPH. It will not go above that. I have performed the following maintenance:

Cleaned throttle body and filter.
Replaced fuel filter in the cool fuel box.
Checked fuel pressure at idle and full throttle. Fuel pressure is between 40 and 50 pounds in both instances measured at the Schrader valve.
Checked throttle linkage from cockpit to throttle body.
Took out old fuel and put in new.
Now going to pull a spark plug to check carbon buildup.
Any other suggestions? It seems as though it is starving for something: Fuel? Air?
Seems to run fine otherwise, just no speed.

 

[Bay B's]

How do your props look? What is your RPM at WOT?

 

[paulswagelock]

Went through this last year with a friends 496. We checked everything, replaced many things, and eventually found 3 failing injectors. Same symptoms, ran fine then next day it had reduced power, gradually got worse and worse as we tried to troubleshoot. We found it by using a Rinda package and killing a cylinder at a time and found it did not affect the performance much when three different cylinders were shut down, but the other 5 made a significant difference. We spent 2 months diagnosing, checking, and changing things.