The term "throttle" is misleading. The foot pedal is actually a RPM request on the governor. More pedal when you are below the RPM is more fuel to get there. However, this is hard to control as you can easily go full fuel due to high load, low RPM, and not being at WOT. Trying to keep smoke down for snap emissions tests is one way to show this. The other is a fixed throttle setting, manual gear selection and engine stops speeding up at a set RPM - at any 'throttle' setting below the governed RPM. The entire point of this is the fuel screw in the DB2 is more of a fuel limit unless you are extremely careful on the throttle. Very hard to tune the transmission shift points to this as well.
Timing: Going advanced increases time to wind up the turbo as EGT's are lower. It also can cause black smoke. Retarded will dump more heat seen as EGT's in the manifolds to drive the turbo. It is possible that you are bringing on too much timing in the upper RPM. The light load retard lever on the side of the pump may need to be brought on sooner. It is possible you need more timing down around 2000 RPM or less to overcome where the boost isn't so high. 5-7 PSI at the IP inlet as anything over that will mess up (advance) timing. Oddly my DB2 likes lots of advance. Black smoke when the cold advance is on and clears when it kicks off with the GM3 was perfect. To me the mechanical pumps rattle more at idle than the DS4 pumps.
Timing explained with results:
http://www.oliverdiesel.com/tech/timing.htm
I have worked on the theory that high EGT's can be from fuel still burning when the exhaust valve opens. More air may improve this. A BD spool valve is one way to do this. Adding a boost valve with the ATT got me the best MPG ever with the 1995 2500 4x4 Suburban of 16 MPG at 75MPH+ Freeway. For me breaking 14 MPG was rough esp with high speed and constant grades everywhere out here. It may hold more heat in the engine than a smaller turbo. You trade off pulling hard all the way to redline with a smaller turbo and then have to mess with getting the WG adjusted.
Humidity has some effects, but after turbo, at 200 degrees and 15 PSI the humidity is way different. Regardless I need less timing with hot dry air vs. high humidity as the humidity slows down ignition and the fuel burn. It was 2% RH yesterday and like 110. It gets more miserable with the Monsoon season - 110 and some humidity.
What ECT temperature does your engine fan start sounding like a jet on takeoff? On flat level ground steady 65 MPH is would cycle on me a few times, but, usually not full blast. The only times I would go slightly over 210 was a sneaky grade for a freeway interchange that did not require a downshift, but, heated it up before the low temp fan clutch locked up. Again the low temp clutch overcomes the delays in getting the slow to respond spring thermal fan to lockup under load. The longer it takes to get the fan on the higher the ECT goes before the temp rise stops. This engine doesn't have much high temp room to play with so I err on the side of cold - T-stats and clutch lockup.
At this point from your past DS4 and my experience with both DS4 and 6.5/6.2 DB2's I suspect the area to look is the pump setup/build/timing. Otherwise you may need to build the rest of the engine around the pump.
