2008 4.3L MPI Merc, RPM issues * please help*

[nd4231]

I have a 2008 4.3L MPI engine in a Four Winns Bowrider boat. I used to achieve 4700 RPM and 45 MPH, however I now noticed that under full throttle, I only get a MAX RPM of 4000, and 37 MPH.
I have made NO prop changes, or other Engine modifications.
I have changed the Water separating fuel filter.
Engine starts excellent, and pulls out of the hole normally. Engine runs smoothly but when I go to full open it seem to become lazy at only achieves 4000RPM.

I have checked while stopped that when I put the control to full open, the throttle plate on the engine is in the full open position.

I am not certain what to check next?
could I have a bad injector? it runs so smooth
Could fuel pump be lazy?
Spark Plugs?
Fuel related? ( I am using 87 oct, never had issues before )

Any ides are appreciated.

ND

 

[WoodieMan]

Simple but got to ask; is the hull clean?

 

[ciscokid]

a dirty injector will cause a power loss and still seen to run smooth sometimes.

 

[ciscokid]

if a injector is dripping and not spraying can have that effect.

 

[ciscokid]

And also like Woodieman asks is the hull clean,will drag the boat down.

 

[Docksidemarineservices]

A few(as few as 5) barnacles or tube growth on your propeller will do the same thing.