If the weights (c-advance) are stuck open (advanced) the engine will be hard to start and run very rough at idle AND run OK when revved up under load. If stuck closed (full retard) it will start and idle fine but very little power and rough running as you apply throttle under load.
Can you twist the rotor and when you did did it spring back? If so it should be considered OK.
Does the engine have points or E.I.? If points the timing will change as they wear however not drastically. With the E.I. the timing only changes if you move the distributor.
Dave Neptune