Timing Advance Sanity Check

Long story short… I have an unmolested distributor with a govenor stamped R11.5 so I should expect to see 23 degrees of mechanical advance with a dial back timing light, right? Other than dirt and grime is there anything that might block the distributor from only getting half of the expected mechanical advance?

Long story long….
I’ve got a ’65 Dart with a Slant 6 that appears to be original, unmolested and parked for almost 40 years. I pulled the distributor to make sure it wasn’t all gummed up and swap out the points for a Pertronix unit. Originally, I couldn’t get the governor retaining spring out so I did my best to clean and oil where necessary. Everything seemed to be free.

The governor is stamped R11.5 which is supposedly good for 23 degrees of mechanical advance, right? I set the base timing to 7-8 degrees (between the 5 and 10 degree marks) and was expecting to see 30 degrees total (w/o vacuum adv.) but my total advance was only 20 degrees. That means I’m only getting about 12 degrees of mechanical advance. Right?

I figured the weights weren’t getting full stroke so I pulled the dizzy again and this time managed to get the governor retainer spring clip out without losing it and did a thorough cleaning and lube job. I also left out the heavy, secondary spring figuring maybe the heavier spring was stopping the weights from hitting full advance.

I put everything back together and didn’t see any change. Now I’m doubting my timing light.

Does anyone know the exact diameter of a slant 6 dampener? I measured my harmonic dampener with a tape measure and it appears to be 7 inches in diameter but I couldn’t get a good measurement with the A/C belt and pulley installed. 7*pi/360=0.061 in/degree . I took some masking tape, a pair of calipers and marked out advance points up to 30 degrees and used the tape to put paint marks on the dampener. I rechecked the timing without messing with the dial back settings and the paint marks seem to confirm that the timing light is calibrated properly. I even grabbed my 30+ year old dumb timing light and it showed the same results.

Before I pull the distributor again, am I misunderstanding the distributor “R11.5” marking? It doesn’t look like anyone has tried to recurve, weld or file on the distributor. Could I have somehow put the distributor back together incorrectly so that something is inhibiting the mechanical advance?

Isn’t it great how the simplest devices can drive you nuts?!