Originally Posted by Frank Reiner

Upon removing the distributor cap, you should be able to turn the rotor (in the direction of rotation) about 15 degrees against spring force. The centrifugal advance weights and cams are located under the breaker plate, and are not visible until the breaker plate is removed from the distributor.
In the event the rotor cannot be rotated, or does not have any restoring spring force, the distributor is a candidate for overhaul.

With a timing light installed, there should be visible displacement of the timing marks with increasing RPM. No movement? Distributor overhaul.