ISUZU JB1 TURBO INTERCOOLER

Friends: I have an isuzu trooper 1989, which came with 2.5 gasoline engine: to which I installed a JB1 TDI; which was brought used from Japan.
This engine is in excellent condition, it works very softly, starts without difficulty (even in winter).
The problem is that it has very little power, in low RPM, less than 2000, it costs you to get out.
I don't know if it is lack of pressure in the turbo, or any of the emission control systems, (which are disconnected) that interfere with performance.
Of course I have already verified that air and fuel filters are clean.
I observe that even at maximum demand, it does not even say goodbye to a smoke shadow for the escape; But I have not wanted to touch the injection adjustments.
If someone has information about it, I thank you for the help you can provide.
Some time ago, I got out of this portal, a manual of that engine, but it was not complete, now I look for it and it doesn't appear anymore.
Thank you so much