2008 Mercury 25 hp won't run WOT under a load

[tinkasochi]

25 hp four stroke runs fine on muffs or in neutral in water but only half throttle under a load. Lift pump is good, vst is good, 45lbs fuel pressure, all new fuel lines and filters, throttle body cleaned, and all electrical connections are cleaned and good.

 

[oldman570]

Depending on the boat and the load it is carrying, the motor might be working to hard because of that. What is the size of boat and the total weight in it? Otherwise the motor might be over propped, which will cause the motor not to reach the recommended RPM for the motor. What is the size of the prop and the pitch?
Oldman570

 

[tinkasochi]

The motor and prop have been working great on this boat since 2008. It had only recently started doing this.