It's the crank sensor!
Join Date: Nov 2007
Location: Central Minnesota
I agree that it's time to take your vehicle to a different shop. Get a fresh set of eyes on it. It is often the ticket to finding root cause.
How have you determined that it is running rich? Exhaust gas analysis will tell that story in a hurry.
As mentioned, base timing is controlled by the computer. Unless the distributor was removed (was it?) or it jumped a tooth (unlikely), this could be something else other than timing.
Seriously, find another shop (with ASE technicians) and let them take a look. The fact that they want to "trim the distributor cap" is a sign that they are in over their head on your problem. That is not likely to be the fix here.
ROOT CAUSE needs to be established methodically, using a testing based strategy.
99 Cherokee, 4.0 AW4, NP242
Past Jeeps: 49 Willys, 81 Scrambler, 88 Comanche
Without "data", all you have is an opinion!