[Excaliber]

Russ, your onto something about a rich fuel condition. That extra potentially unburnt fuel would more easily re-ignite with the introduction of fresh air via an exhaust leak in the system. End result, "popping" on decelleration which MIGHT go away by simply adjusting the fuel/air ratio. If it's rich enough, it might self ignite even if you didn't have a significant exhaust leak allowing extra fresh air into the system.