[justcuz]

It is probably timing. What is it set at initial? It should be somewhere between 4 and 10 degrees. Need to check a book if you have one. 8 is a good place to start. Basically if you are going up hills the vacuum advance is supposed to allow you to keep a constant speed without the pinging. As you accelerate vacuum drops retarding the timing as you climb the grade. If the vacuum advance is working you have too much advance. You can check the vacuum advance by using a vacuum pump on the advance mechanism and seeing if it moves with the distributor cap off.