First off I don't have a whole lot of info. because this is my nephews car, but any how we have a 1986 mustang with a 4cyl engine that idles very rough until he gives it some gas then it smoothes out. I know this isnt much info to go off of but can someone give me an idea were to start PLEASE.
Is it fuel injected?
I would check the idle air by pass port. Its a control that opens and closes to let air by pass the throttle plate controlling the idle speed.
It could be dirty. It's bolted to the throttol body. You will need to get a replacement gasket but it won't cost much. I would also remove the throttle body and clean that as well. You will need a gasket for that too.
These are cheap parts and easy fixes.
It may not solve your idle problem but it can't hurt.