Small Block & Big Block Chevy Specific - K5 running rich after tune up

OK guys Im a full time mechanic but used to working on imports honestly. I got suckered into working on a 1989 Fullsize Chevy K5 Blazer 350. This guy was supposed to do some drywall work at my house, on his way here his truck died. So he had his neighbor work on it and found it needed an ignition control module. So they pulled the distributor and replaced the module, but did not set the timing correctly. It toke them a week to do these repairs and he finally gets to working on my house. In return he askes if could do a tune up and set the timing on his truck. So I agree and do the complete cap, rotor, wires, and plugs. All name brand parts and gapped the the plugs to .035 as marked on the emissions tag. Also set the timing to 0 by following the tag and disconnecting the tan/brown wire that control the timing advance. After everything is done he drives and says it fine for about 2 days then starts to run rich, enough so he can smell it coming through the rear window when driving.

So I check timing again and it exactly how I had set it, but I can smell it running on the rich side. All the plugs are tight, and wires are still connected.

Any thoughts on this? I was thinking it was a crappy ignition module or the coil is weak now. If they put the distributor in wrong it probably wouldn't run at all.

Is it a coincidence or did I do something wrong?

I set the timing to the larger "V" on the timing tab

I'd check the coolant temperature sending unit. If this is bad (or unplugged by accident) then the computer "thinks" the motor is cold and dumps more fuel to help it warm up. It is possible that the computer never puts the engine in closed lop mode at all. Just my .02

i have an 85 K5, i honestly do not know much about the tbi injected units when it comes to this, hence why i stay with my carb.

but i can say this for a fact, my truck is dialed in very very close. once i got it built i had my friend thats a chevrolet certified tech dial it in/tune it ect...
when my rear window is down you smell the exhaust, thats just how it is, the rear window down sucks in air to the cab. my exhausts exits in the rear like most of these trucks do and it all gets sucked into the cab. i dont have a/c and with it being hot out the rear window has to come down ( heat index here has been from 115 - 105 every day the last week ) and i just have learned to live with it.
just my 2 cents here.