The Stovebolt Site Forums The Shop Area Engine Shop 1972 350 - backfires at shutdown

If the carburetor is a Rochester 2G with manual choke and a centrifugal/vacuum governor, I would temporarily disconnect the steel vacuum line between the distributor and the carburetor and plug the line at both ends. This will allow you to set the idle speed and mixture without any unknown vacuum leaks affecting the settings. Test drive to heat the engine to full operating temperature and see if the backfiring changes. You may also have to disconnect and plug the main vacuum hose that powers the HydoVac and 2 speed in order to diagnose a vacuum leak.