I just tried to adjust the timing on my relatively stock 79 with original 12a and points ignition system. From what I've read, a good place to set a 12a is leading 24 BTDC and trailing 16 BTDC with vacuum advance disconnected and the engine at or above 4000 RPM.

I notched and painted my pulley and got out my new timing light. I set the leading just fine. I moved the pick up to the #1 trailing plug wire and tried to adjust it. The trailing timing would not move. I could physically move the vacuum diaphragm but the timing would only move a couple degrees around the 24 BTDC that I set the leading. I turned the car off and took the distributor cap off. By moving the trailing adjustment I could significantly move the position of the trailing point but this would not reflect on the timing marks.

I eventually want to upgrade to an electric distributor by that isn't in the cards right now. I am trying to make my points system work as good as possible for now. Any help in explaining why is this occurring, if I am doing something wrong, or if there is something wrong with my car would be appreciated.

 

Use the procedure in the FSM instead. It works. 0 and 20 @ idle, adjust leading first.