1974 Vespa Sprint – popping & hunting on decel

Trying to dial in the carb on my ’74 Sprint and getting a lot of popping and “hunting” when rolling off the throttle. No matter how I lean or richen the mixture screw, it crackles on overrun and takes several seconds to settle back to idle. I’ve checked for air leaks and haven’t found any so far.

Am I chasing a lean pilot, rich pilot, or something else entirely? Which way should I be turning the mixture screw on a Spaco/Dell’Orto carb to calm this down?