Anywhere from 3-7 psi will work, 7 is pushing it.
I know '80 2.3 carb has the metered power valve and maybe the '79 as well and you may be loading engine far more there than a car's weight ever does. You may need 100% power valve open, the later carbs are metered for emissions and do not do that. Way to know is type of power valve assembly, you can tell the carb from top, the early ones have a bi-symmetrical top flat where air cleaner mounts. The later one is not, it has a kickout on one side for the changeup in the car casting for that metered power valve or the later modulated one used on feedback systems for emissions. Those slowly open rather than either 100% open or shut like the early one does. Vacuum hose routing on carb to power valve area must be modded or at least very closely looked at as well or power valve will not work at all.
I ran into this once on my '80 2.3 while trying to find a severe mid rpm lean-out issue, it turned out the later power valve vacuum circuit was not opening the power valve I had installed. Once fixed it ran perfect.
Lesser weight (load) of car allows engine to still run with leaner than crap power circuit, bigger load of skidsteer may well need 100% power fuel all at once.
You can tell the metered power valve that screws into bottom of bowl from the off/on type, the metered one has a long slope on the center valve, the other one simply pops open or shut.
'We beleive the timing is set correctly, but anything can happen in back yard mechanics..... ......'
'.........after adjusting the timing until it gets loaded up.........'
Nothing personal but this sounds truly like back yard work and flat unacceptable. If I told you all the engines I've seen with holed pistons from 'experts' guessing this.......... .no insult intended at all but if guessing at the timing then get a light and set it correctly and probably your issue alone. One either knows EXACTLY where the timing is, or they don't and lost there. Need at least to check for proper centrifugal and vacuum advance, that alone could be the issue again. Make sure you are using the correct port for vac advance.
The new 'CDI' (it's not you know, simple inductive electronic only) box must have a blue grommet where the wires go under box. Other colors can mess with your timing in big ways, they are specialized for certain applications and coded by the grommet color. Just sayin' in case someone quickly snapped up one at the junkyard.
Many V-8s will seem to run fine with a certain plug wire disconnected as only 1/8th of the power is missing, they can even seem to idle smoothly, not so a four, it will be missing 25% power and readily noticeable.