Low Engine RPM's

I assume the observation that you are not reaching WOT is based on something other than the tach?

All of the symptoms point at the engine not getting fuel. That could be caused by a laundry list of issues including a control lever slipped, a fuel filter almost blocked, governor issues ...

One important thing that was missed and will help with troubleshooting - does it reach full rpm in neutral?

Any fuel filter vacuum gauges? Observations?
Have you searched any (of many) similar threads already posted?

I'd look at fuel filters first. If its been a while, change them both....(primary+secondary). Then make sure it's actually getting full throttle at the rack. Also make sure the air filters are clean. If that's all good, I would think the turbo would be the next thing I'd look at.

Both of those items will lead to black smoke unless there is a charge air pressure compensation feature on the governor. If so that is easily tested. Considering the OP clearly stated "there is no smoke coming out of the exhaust" it is fairly safe to say that charge air conditions may be placed far down the list of potential problems.

That leaves fuel starvation as the most likely scenario. Either the engine is not getting the memo about how much fuel to deliver or it is not getting the fuel that is called for.

If the engine reaches and can maintain high idle in neutral it is most likely a fuel filter issue.

What rpm will the starboard engine turn at idle? Will it go higher than 1700rpm?

Should see a no load high idle @ 2300 rpm.

If you're reaching full RPM in neutral I'd see if the prop were fowled or the shaft bound.

There is quite a difference between neutral full rpm and full load rpm in amount of fuel required to achieve each level, If the prop were fouled or shaft bound there would be no shortage of fuel and therefore one would expect a wee bit of puff, the earlier statement that the revs fluctuate ( hunt ) by 200 rpm is also an indication of fuel starvation.

Agreed, but more info is needed, such as what happens when he brings her up in neutral. Also, I didn't get a clear statement about whether that 200 rpm fluctuation is when he's running manual or on sync. We had the same fluctuation, and I changed Racors twice before it dawned on me that the boss almost always ran synced. When I checked I found it was a problem with the sync (not yet resolved).
While we're waiting for the OP to come back, and since it may be relative if it turns out that he's got two separate things going on, any thoughts on what would cause that with the sync?

Loose wire , dirty connections or chips on the speed sensor if its of the screwed into the flywheel housing type

Glendening stuff on old Detroits. #1; engage cable slipping. Read Glendening support instructions on how to tighten it up.
#2; Mechanical tach cables whipping. Pull the tach cables, make sure there is not any bends, pour in 5 drops of oil and re-insert the cable from the same end you just oiled.

Thanks guys. I've never done diddly with an auto-sync, in fact rarely use them (prefer my ears). So today I learned something new. Another good day.

Assuming a Glendenning, it could be the flex shaft in the tach cable drive adapter. Past experience with an 892 it would wear down and tach stops working properly and then the synchronizer doesn't know what is going on. I think they use the same/similar configuration.

Your not over fueling or under load.

Suggested by a s$#%^&^.

The return line will not cause a fuel starvation issue.
Detroits use a restrictor at the left head already, say 0.080 (or less).

Per your description of clear exhaust; you have a rack (governator) issue or more likely a fuel starvation issue.

You may have already trashed the injectors. They have their own lil filter.

Anybody pull the pickup tube yet.

No response yet on vacuum readings or pressure readings.

Read any of the other threads yet?

Or believing what someone suggested in blowing out the return lines...

A vaccuum gauge on the racor (which is good to have anyways), or anywhere in the fuel supply line will tell you if you have a fuel starvation issue. A worn turbo would cause a loss in power. Are you sure that it's getting full travel on the throttle cable? I don't think the return line has anything to do with your issue.

Usually the skipping tach (or even low reading) is the tach pickup needs to be adjusted at the flywheel. But also the cable going to the glenndenning is another popular source of that issue.

Give that person bagpipes to blow.

Only if it is downstream of the lift pump. It won't tell you squat about what is going on in the delivery side. Vacuum gauges are for monitoring filter condition or tank valve status and that is all they are useful for.

And a great deal of smoke if the fuel metering and delivery is functioning normally.

No one said the rpm indication was "skipping" it was described as "slightly fluctuating between 1500-17oo RPM" which lacking further clarification normally means a smooth increase and decrease between those points. This is what can be expected if there is a fuel starvation issue.