Vacuum advance

Hello all,
I have a 1970 standard conv., 351 Windsor, automatic, California emissions. I have a question about the vacuum advance. I have a thermostatic switch in the vacuum lines going to the distributor. The distributor has a dual port advance. I believe one is for advance and the other retards. The vacuum lines going to the switch are manifold vacuum and ported vacuum. Can someone go into detail about how this system works such as what tempature should allow ported or manifold vacuum to which port on the advance please?
My base timing is set correctly and the mechanical advance works properly but I have a lot of detonation/pinging issues while cruising.

Gasoline today is not as good as it was in 1970. Your distributor originally had 40 or more degrees of total advance. You can change the limit in your centrifugal advance to fix that problem. Or buy an adjustable vacuum advance from a specialty website like

The temp sensor increases advance under idle conditions to make the idle speed increase which in turn causes the fan to blow harder which causes more air to move across the radiator at idle in hot weather.

So the outside port should have manifold vacuum while idling? What about the retard port, when should it see vacuum?

I believe the answer to your question is in the shop manual. I don’t have a manual for 70’s but earlier manuals talk about it However, the thermostatic switch in the thermostat housing is extremely durable and is not likely your problem. My 69 351W had a pinging problem even after a total engine rebuild. I checked everything and could not find anything wrong. Like Royce, I felt is was the lower quality of gas. Much later I installed a Pertronix ignitor and the pinging went away. If you still use points, it might be worth a try.

I already have the Pertronix. I tried 91 octane over the weekend and it seemed to help alot but it was still there slightly.

None of the small block Fords I’ve owned over the years have ever liked much advance. I’ve always swapped out the stock advance canisters with aftermarket units so I could back off the advance. Heck, the factory specs for my 68 J code (10 1/2:1 compression call for only 6 degrees of initial advance-- and that was back when rocket fuel was available at the pumps. Maybe I’m doing something wrong, but I’m amazed at the number of times I hear folks recommending 10-12 degrees of initial advance for cars with vacuum advance distributors. They must either not drive much or have access to much better fuel.

If you read the section on ignition timing the shop manual actually tells you to set the initial advance between 0 degrees and 12 degrees depending on altitude, gasoline available locally, etc. The specifications section does indeed say 6 degrees.