1992 Stanza: won't start in the cold

My grandfather has a 92 Stanza and whenever it's cold out the car has trouble starting or won't start at all. It also emits a black smoke that leave a soot-like residue.

He constantly has to change the spark plugs, then the car will start okay for a week or two, before not starting again. All the while the car continues to emit the smoke.

We've brought the car to a lot of different car shops, including the local Nissan dealership and no one has been able to figure out what the problem is.

If anyone can offer any advice or suggestions, it would be appreciated!

Thanks.