Volvo Penta low power

[Rufduck]

A bit frustrated this year with my Volvo Penta diesels. D1-30B
Port engine at WOT is down to 2100 rpm from 3200 in gear and under way and now the starboard engine is only turning 2500 under same conditions. Both engines exceed 3200 revs no load.
Both engines seem to run fine up to the revs indicated as you advance the throttles. Beyond that point more throttle does nothing.
Observations...
Neither engine smokes at WOT. BUT port engine burps out soot on cold startup.
The throttle appears to move the throttle arm on the injection pump to full throw.
Each engine draws from the same common tank but have totally independent fuel plumbing, filters etc.
Saildrive oil is clean and unburned. Neither runs hot.
Both primary and secondary filters replaced or cleaned.
Same props for about 2500 hrs. Three blade folding.
Visual inspections of props for function or damage show nothing amiss.
No excessive vibration on either engine.
I even plumbed one engine direct to a separate fuel tank as a test for possible fuel blockage and tested last night. No change. The experience was educational and nearly embarrassing, however.
I Just removed the port air cleaner but was told that the engines would smoke if air limited. Also, they are more silencers than filters. No change in performance.

Getting pretty low on options.
Exhaust elbow where water is injected into the exhaust to cool it?
One or more Injectors partially blocked or not passing enough fuel? OR nozzles not atomizing fuel properly?

This sound familiar to anyone? Any thoughts really appreciated.

Thanks.

[Lepke]

Most likely causes are not enough fuel or air at high rpm or a restriction in the exhaust. Do the engines have access to good airflow? Some people to quiet diesels have too little air flow by closing or restricting vent ports. Exhaust elbows becoming clogged with rust and restricting exhaust air out of the engine is a common problem.

[Wotname]

Hmm... my previous reply failed to show up - let try again .

I would try a couple more easy things before worrying about the injectors.

Pull and clean the exhaust mixing port and confirm exhaust system is free of partial obstructions.
Check / adjust valve lash (tappet clearance)