turn up the fuel on the 89-93 6BT's
For the 89-93's (Bosch rotary pump)there's a screw behind a tin access cover about the size of a nickel. It takes a 1/2"(13mm) to break the jamb nut loose, then take the screw out. It'll have a metal band welded on it that needs to come off. Once that's off you can run the jamb nut farther out(towards the outside of the pump, about 2 turns). Then put it back in and test, some smoke more than others(especially with Lucas POD injectors!). The pre boost power adjustment is under a cover that looks like a vacuum diaphragm, and it is one. It has a torx bit with a jamb nut. Loosen the torx for more low end & tighten for higher. The governor spring is different than in the later 12 valves, and is not adjustable.
The other adjustment is to rotate the diaphram, under the top cover with 4 screws. Use the picture at right as reference. Turn the diaphram clockwise 90-120 degrees, this is like sliding the fuel plate on a 94-98. Now put it back in & test.
The best bang for the buck, is to get a gov spring, most of the time they are under $20, and let it rev up to 3200 rpm. If you plan on a gov spring, hold off on doing the fuel screw shown above, it needs to be close to stock setting. Great instructions are here: