Loss of power at 4000 rpm

[Lakeview]

I have a 4 stroke Mercury 60 EFI, Serial Number 0T755241. Without a load on the engine it will run up to 5500 rpm, when I run it with a load it will not get above 4200 rpm and act like it wants to die. I have been going through the fuel system to see if I could find the problem without success. I have changed the fuel line and bulb, changed the water separator filter, cleaned the fuel injectors, ohm tested the fuel injectors, (all tested at 12.6), and installed a new high pressure pump. There was what appeared to be rust on the old high pressure pump and in the bottom of the VST. VST was cleaned and air was blown through the low pressure line with the float out attempting to clean any trash in the line. I have not changed the high pressure fuel filter after finding all the rust in the bottom of the VST. Doing research I found that the voltage regulator/rectifier could be an issue and tested it at idle for power and it was reading 14.2v. It still has the same issue with power. I cannot get it to go to WOT with a load. Can someone give me a hint as where to look. Thanks in advance.

[EuropeanAM]

Welcome to BBC!

Moved your thread to the 4-stroke Forum for you.

I would not continue throwing parts at it, as it's quite likely that you'll end up putting an awful lot of needless money into "guesswork".

One item we've seen with this series is that the o-ring that seals the MECHANICAL fuel pump shaft tends to get dry and hard, resulting in the shaft not returning quickly enough between strokes. REPLACING that pump is the best method, though for short-term "proof" of the problem I have seen the pump disassembled, o-ring and shaft cleaned, 2-4-C grease applied and then pump reassembled long enough to make a good run and prove that it made a difference.