Ok so I have an 84 lx convertible. It has sat for 3 years, was running when parked. It will start but not run. It was supposed to be my 16 year old's car but he now has a scion TC( wife reliability and safety issue) I would like to get it running and out of my yard. I have a 90 lx wiring harness as the 84's wiring looks like rats have eaten well for the past few years. Part of the reason it fell in disrepair is because I lost the use of my right arm and now take handfuls of drugs. I need to get her back up and maybe this will help me get over the disability. Any help is appreciated. Thank you.
The 1990 wiring harness won't work with anything but a 1990 Mustang. It's an odd duck because it has some wiring connectors that don't match any other year Mustang. On top of that all the cars after 1985 were EFI.
Your car is carb, and none of the later wiring harnesses will work unless you plan to do an EFI conversion. That may be beyond what you want to get into.
I suggest that you drain & clean out the fuel tank and fuel lines. Plan on replacing the fuel filter and putting an overhaul kit in the carb while you are at it..