Distributor curving.

Backfire through exhaust is late ignition timing. Make sure you put the reluctor back on correctly- assuming it's electronic ignition. If points, check your dwell. Make sure all your electrical connections & primary ignition wires are good too. I'm also assuming that you didn't change anything else while you were in there? No changes to carb etc? Plug the vacuum advance back in too & see if that changes anything.