yeaht, as noted above, it sounds as though your idle advance diaphragm is torn/rotted.

according to the FSM, the Idle Advance works off of Manifold Vacuum, and is supposed to advance the timing at low to no load conditions by 10-14?. the Normal Advance works off of Ported Vacuum through a port on the carb above the throttle plates.

I ended up replacing the entire distributor. got a new distributor from Advance Auto complete with vacuum advance module for less than the cost of the vacuum advance alone. still working after five years or so. if memory serves, it was around $130, and worth it for the increase in drivability/throttle response, fuel economy, and for smoothing out the engine in general.