328 backfire

For some time now (at least 2 years), when I turn of the car, it will have a slight backfire. My catalytic converter was gutted several years ago and it did not happen after gutting it. I have tried running a leaner fuel/air mixture and a richer fuel/air mixture but it does not seem to make a difference. I have also run the idle less and run it higher and that also does not make a difference. What could be causing this? The car runs just fine although, I have to give it gas to re-start it after it has become warm. If I just turn the key, it will not start when warm or hot but giving it gas will start it up. It always starts up without giving it gas when it is cold.