I have a 91 3.0l ranger that i had to replace all of the injectors on. When I pulled the injectors I noticed that all of the ports were all but closed with carbon.
I cleaned the ports , replaced injectors , replaced map sensor and the tps. I proceeded to time the motor and when i replaced the in harness plug (don't know the name)
after timing the motor , and the timing seemed to drop. I connected the light back up and I was correct. I don't know what to do to get it right.

 

Welcome to the forum

1991 3.0l Ranger uses a TFI spark system
TFI module on the 3.0l engines was remote mounted, not on the distributor like 2.9l engines
TFI module should be on drivers side at the front of engine bay mounted on the Rad support metal piece, under Battery

Near it you will see the SPOUT connector, looks like this: https://www.muscularmustangs.com/images/timing1.jpg

2 wires, the plastic end pulls out, this disconnects computer from TFI module so you can set base spark timing without computer interference

Engine needs to be warmed up to set spark timing
After spout connector is removed, start engine and set base timing, spec is 10deg BTDC, some prefer 12deg BTDC
Once timing is set tighten down distributor, check timing again, then shut down engine
Reconnect Spout
Start engine
timing should now show 18-22deg BTDC with computer advance added

Computer advance is what replaced Vacuum Advance used on older engines, and you disabled vacuum advance to set base spark timing in those days, SPOUT connector disables computer "advance" so you can set base spark timing