I realize the injectors don't screw into the block, brain fart.
Agree with you that the injectors were manipulated by previous owner, which is why I suggested making sure that they are correct by s/n. Just because they screw in doesn't mean that they are, there are pressure and fuel volume considerations.
After looking at the pictures again, it certainly does look like the one on the right
was sealing less well than the one on the left. Would that cause the symptoms described? Maybe. By all means clean the seats well when reinstalling the injectors, and use the parts
and procedures for the specific model and s/n engine you have per the manufacturer. (Had a Volvo
once that the pump shop said seal injectors one way the Volvo
manual said another way, turns out there was a change made by Volvo that wasn't efficiently communicated. I went with what the pump shop said and had no problems, found out later about the Volvo change.) Keep it clean as possible, but don't worry too much about a little carbon falling into the chamber.
That the engine sometimes runs smoothly and at other times doesn't (as well as the good compression numbers) indicates that there is probably no problem with the rings, valves or head
sealing. Likewise with the injection pump and governing mechanism.
So we go to fuel supply. Again make sure the filters are seated well and there are no intake side leaks
that would allow air to be drawn into the system. The fuel tank
pickup could be clogged (I had a similar lack of high end rpm issues once, some moron had put a strainer on the end of the pickup tube in the fuel tank
. Cut it off, problem disappeared.) Also make sure the vent line for the tank is not obstructed, very important. Check the lift
pump too, an inline fuel bulb pump can sometimes be used to identify a faulty lift
pump; when the engine starts missing does pumping the bulb eliminate the miss? Maybe not so much with a tank vent or pickup issue though.
What make engine and how many hours?