87 Trooper Backfiring

Odd problem. Just replaced the cap/rotor, spark plug wires and spark plugs. Just purchased the car a few months ago. Seems as though it was poorly taken care of. Spark plugs looked really ancient. They were a pain to get out. One of them broke in 3 places removing it. Anyway, got them out and replaced them. Went to start the car. Car started up but now it won't idle on its own and when I let my foot off the gas it backfires eventually dyeing unless I rev the engine again. Any idea why its backfiring and not idleing?