[rotorwrench]

Any time the points plate moves when the cam is motionless, it will change the opening gap since the cam spacing it so close with an 8 lobe cam. Any play in the pivot movement of the points plate could be problematic too. The vacuum advance will move the plate but centrifugal generally only moves the cam. If there is any play in any of these movements then that would be problematic as well. Many of these old units never get any lubrication service and stuff does wear out. Even the cam can get worn out since a lot of backyard mechanics never even apply lubricant to the follower wick. If the points plate is not concentric, it is likely defective.