Timing Advance Sanity Check

[blind bat]

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?!

 

[Professor Fate]

Advances are marked in distributor degrees, which is half of crankshaft degrees; which is what you're measuring with the timing light.

 

[Bewy]

My MM manual that covers 65 models shows centri weights of 10,11 12 *. None listed are 11.5*.
Slant 6 engines list max centri @ 2200 [ 4400 crank rpm ]. So checking for total centri adv on a stock dist needs to be done at high rpms.

 

[1WildRT]

Measure the length of the timing advance slots... If they measure around .428 you timing light is suspect... If they measure .340-.350 somebody may have modified them... Honestly less mechanical allows you to add more base timing which generally helps with low rpm drivability...

6 deg 12 0.340
7 deg 14 0.357
8 deg 16 0.375
9 deg 18 0.390
10 deg 20 0.405
11 deg 22 0.420
12 deg 24 0.435
13 deg 26 0.447
14 deg 28 0.460
15 deg 30 0.475
16 deg 32 0.490
17 deg 34 0.505
18 deg 36 0.520

 

[blind bat]

Bewy - The piece with the 11.5 marking is above the weights. It’s the part the weights are pushing against That has the points cam. The advance seems to stop around 2200 RPM.

1WildRT - I’m kicking myself for not measuring the slots while I had it apart. And I agree, I probably shouldn’t be looking a gift horse in the mouth. Being a Mopar noob I did a bunch of research before taking the distributor apart and not seeing the results I was expecting had me questioning my understanding of how the mechanical advance works.

 

[4spdragtop]

Hook up a Mighti vac to vac advance to check.
Edit....
Isnt the stamped #'s relate to vac advance not mech advance?

 

[Mattax]

Not once have you mentioned the rpm which the timing was measured.
You need to measure timing at the rpm listed in the FSM (x2 on the engine) to see the maximum advance.
More important is to measure the rpm at slow idle, then the next rpms listed in the FSM (x2 on the engine).
Then you'll now the advance is within the parameters for that distributor.

I've posted a few graphs illustrating this for slants.
edit: Here's one: Dizzy curve

 

[Mattax]

me questioning my understanding of how the mechanical advance works.

A lot of times changes do not do what people expect. The more you work with the distributor the better you will understand the mechanisms. The governor's upper limit is not the most critical item even though people think it is.
We step through much of it here.
Distributor curve help needed

Summary of all of the contrlling functions here.
lets revisit timing