update.

I followed Crizila's advise.

I pulled the nut off the vacuum advance pot and pulled the spring. When I had the distributor rebuilt 10 years ago they custom cut the spring length. There were no washers or spacers.

I added 4 washers totaling 0.240" which gave me the following results:

(Factory Spec. = FS)

FS – 0 @ 6-9 inches
Actual - Advance starts @ 9-10 inches of vacuum

FS - @ 12” – 4.5 – 7.5 distributor degrees
Actual - 4.5 distributor degrees

FS - @ 15” – 8.25 – 11 distributor degrees
Actual - 9 distributor degrees

Unspecified - @20” – 12 Distributor degrees

Total advance @3500 RPM with no load on motor 56 engine degrees (This is 2 degrees less that it was before)

The original distributor settings were used for 10 years @ 12.5 degrees of initial and when I pulled the motor apart for this rebuild there were no signs of detonation on the pistons, heads, valves or spark plugs over the years.

With the new pistons compression is down 3/4 - 1 point and initial timing is up by 3.5 degrees, but still set at 32 total mechanical.

So I am going to leave it as is and listen carefully for any sign of pinging. The next time I have the distributor out I am going to drill & tap the vacuum advance arm and put a screw and an adjustable stop on it. (A small piece of sheet metal with a slot in it) to limit it to about 50 degrees.

I really appreciate all your help with this project.

Thank you again for taking the time to input.

John