Low Engine RPM's

From both engines or the stb?

Could be. Without a pressure gauge installed, nobody can tell.

A vaccuum gauge on the supply line at the racors or just after the racors would show if there is a restriction on the fuel supply.

As for the tach, check the pickup that is mounted near the flywheel. I've seen a lot of boats equipped with Detroits do this and a slight adjustment on the tach pickup usually fixes it, or replacing the tach pickup.

I'm not exactly sure how Detroits are configured, but do the injection pumps have a screen in them like the common rail mans do? There might be some algae stuck in there. Do Detroits have a mechanical lift fuel pump?

DD's have a Gear Pump to provide fuel to the Injectors. The pickups in the flywheel housing may well have chips off the ring gear /starter pinion and rust on them so are well worth a clean, make sure you get them right back in the same place (depth) in relation to the flywheel, too far in and the teeth will rip the end off , too far out and they won't read properly.

The one thing they won't need as such is adjustment - There ain't any.

8V92s usually draw from the tank, thru the racors, then the gear driven lift pump, then the secondary fuel filter, splits there, then to each head. The return line comes off the right head, to a T on the left head, flow restrictor, fuel cooler, then back to tank. I have found the restrictor on the return on the cooler on some 12Vs.
Without a pressure gauge on the secondary filter, you won't know if the pump is up to snuf.
Naturally, poor fuel lift pump, no low pressure to be recorded at the mud filters making you belive there is no restriction, BUT you have low pressure and/or flow at the secondary filter and injectors also. These lift pumps are very dependable but like anything else, can fail. That's why it's important to include a pressure gauge at the secondary filter.
Each injector is cam driven to deliver it's high pressure fuel delivery. No high pressure injector pump in the fuel supply like a Cat & others.
Depending on the injector (N, S or exoctic), stock detroit injectors do have a filter inside.

I was always told by DD mechanics to bottom out the pickup, then back them off 1/2 a turn.

That is if a mag pickup is used at the flywheel. Pretty sure there is not a port in the housing for it. Very not likely on an old Hat with 92s.
Typically a take off from the back of the blower drive, drives the tachs. A mag pickup is there or usually at the glendening syncro box. If it's at the blower drive, then it's the motor varying rpm, if at the syncro box, usually the tach cables.

The unknowns are still unknown ... does the engine reach high idle rpm? Does it really change rpm? Is the rack capable of full and free movement to the limits?

And, as K1W1 mentioned, fuel pressure upstream of the lift pump is critical, does it meet specs? (around 80psig max - look it up)

Need a few days of patience.

Do you know what your fuel pressure is when running at High Idle in Neutral and when under load at wherever it gets to?

What you've described so far really points to a fuel starvation issue, you will need a lot more fuel to go at full rpm under load than you do in Neutral, this would lead me to check the pressures or ideally the flows of fuel, you might have chocked secondaries or a defective fuel pump - those gear pumps are very reliable but sometimes do give trouble. Some of the gunge that was in your fuel might also be restricting the fine gauze filters in the injectors.

I agree with K1W1 that it sounds like fuel starvation. However what RPM's exactly and were these verified with a digital tach? DD's have a lot of droop and should reach over rated RPM at high idle.

It is a simple check to put a fuel pressure gauge on the engine. That's the easy place to start.

Is the boat still limping or did you fix it?

Actually since the tach does not vary in RPM's with no load, but does so with load. I'm almost thinking that the tach is accurate and the fuel system has an air leak somewhere on the supply side. So when the motor is calling for a lot of fuel, it's getting air bubbles, dropping rpm's, and then picking back up again. It definately sounds like it's on the fuel side and also on the suction side of the lift pump. Somewhere between there and the tank.

How do you get an air leak into the fuel system without also getting a fuel leak out? I'd be more inclined to think in terms of either a fuel or air blockage, a pump problem or even a throttle cable slipping.

Any leak above fuel level.
Bad O ring on top Of a Racor with fuel tanks below is where I usually find it. Bad draw fitting on the tank, bad hoses, cracked fittings, failed clamps & fatigued flares.

Wouldn't all of those things allow fuel to leak out?

DD's fuel is under vacuum until it passes the engine fuel pump so a minor leak would rarely show up .

No, the fuel wouldn't leak out. I've also experienced push lock hoses that under a lot of suction would allow air to leak, causing at cruise, you hit a wave a little, fuel line was allowed to jiggle a little and it sucked air and you'd get a 2-300 drop in RPM's and it would pick right back up instantly......yet at slow speeds no fuel leaks, no running issues, nothing.....it took 4 MAN mechanics to figure it out......

NYCAP- it's like if you have a garden hose and it's under pressure you can sometimes grab it 6" from the hose barb and pry it sideways a little and all of a sudden it leaks some when it didn't before.

To the OP- I'd borrow a fuel line and run it straight from the tank to the lift pump and seatrial it, and see if it changes the situation. I've also seen Racor petcocks that would allow air in but wouldn't leak......A lot of times you can open the petcock up all of the way but it won't even drip until you crack the lid and break the siphon.