91 cavalier runs lean after warms up

Hi, I hope someone can help. I have a 91 cavaliar 4 cyl with tbf that starts great runs great for about 15-20 min.when it gets to normal temp it will start running rough , black smoke comes out the tail pipe, it runs real lean and will die. It will not start again until it is cold. any ideals. Thanks

i would say its a o2 sensor. it gets up in temp and computer reads from the sensor telling it to run richer.