Distributor curving.

The more i look at this, i may be off base here. My thinking was that it's unburned fuel making it's way to the exhaust. With the vac. adv. hooked to ported vac. you lose that advance as soon as the throttle is closed and then the mixture is burned later in the cycle allowing it to be closer to the exhaust. The fly in the ointment is this is not happening with the vac. advance unhooked, which simulates no vacuum on deceleration. My theory was that with manifold vacuum, you still have the extra advance during deceleration and would burn the fuel before it got to the exhaust. Since this just occured with the recurve on this dizzy, it could be something completely different. If by chance the manifold vac. cured the popping, the initial timing may have to be backed off several degrees to run the manifold vac. and not be over advanced in a normal cruise situation. Hope my thinking made sense to y'all, if not, let the beatings begin...lol. :violent1:.