Hi zackery the screw you are talking about is the fuel load control and it is very sensitive ,and it normally needs to be set on an inj pump test mach, 1/8 turn from the fact setting will result in extreme smoke one direction and the other a loss of power, and robt was correct in that dsl kiki and isuzu sealed all the adjustments so that you couldn,t change them too easy,, what i have done in the past to adjust one that has been miss adjusted is warm up the eng make sure ign timing is right and the adjust idle speed up to about 1500 rpm loosen the lock nut and very slowly turn the screw clockwise untill the eng starts to die down in rpm and then start to turn the screw back the opposite way untill the spd comes back up to about 1800 and lock it there if it hasn,t been turned too far from the orignal setting you can get it pretty close , i have one here rite now that was turned in about 3 full turns and it smokes like a freight train , the screw just pushes against a small lever inside the pump that controls the pump cam position which is what pushes on the disttributor pressure piston which in turn produces the pressure and the vol, and it can be damaged by too much twisting , i am about ready to pull this pump and send it out to be recalibrated , too much time messing with it , you can tell the pump guys how much increase in fuel del, you want in percentages and they can set it dead on, but then the increase usually means a little more power and less fuel mileage , because the customer tends to use the pwr more , Jerry oh if you want a quick and easy upgrade so that it will advance the timing quicker, here is an easy one , on the side of the pump there is a timing advance mechanisim, it is called a cold start advancer , but has nothing to do with temp , it is pressure operated, look down on the side of the pump and you will see a large round , looking nut with about a 10 -12 mm hex rite in the center, unscrew this cap and take out the large holding spring and any shim if any , and then screw the cap back on , dont pinch the oring and the timing will advance much smoother and quicker as you go up thru the gears and it will eliminaate any of the low speed, mid range eng lay down in 1st and 2 nd gear, over all runs much better