The movement of the distributor housing from the action of the vacuum advance can break strands in the wire from the negative coil terminal to the distributor, causing the coil primary circuit to open up intermittently. The result is like switching the ignition off momentarily, then back on. It will either cause a backfire out the exhaust, or occasionally a pop back through the carb.
Disconnect the wire from the distributor side of the coil, connect an ohmmeter to both ends, and stretch/twist it. See if the wire goes open circuit with the right amount of flex. It's a problem that can go unnoticed, and cause you a lot of grief.
Jerry
