There's a great write up on Steelsoldiers on how to get to the fuel adjustment nuts. You will be happy after doing it.

About to do some tweaking on my 91' M939 series. I'm going to isolate the aftercooler first and put it on it's own radiator, right now it's a total dog if it's any kind of warm outside. When it's cooler out, the thing is respectable. It also needs more timing and fuel though, definitely. Good to see someone else playing with the military trucks!