timing keeps advancing

[sleeper bird]

ok folks iv got a weird thing happening 66 q code,my timing keeps advancing on its own,i cant physically move the dist when its locked down,set it at say 6 and it advances to 12 within 20 miles of driving.Its got a petronix unit everything is tight and not moving im bewildered as to what is happening.

[georgemii]

The centrifugal advance occurs compliments of the weights.
As RPM increases, weights advance the cam; in your case, the Pertronix magnet.
Magnet is mounted on a shaft which is independent of the (lower) driving shaft.
Upper shaft should move freely on the inner/lower shaft. See oil wick at top.
It is possible that upper shaft - or the weights themselves - are hanging up in
the advanced position. Or broken return spring/s on the weights.

My $0.02 worth.

[David Langhorne]

I can't see how that would affect the timing as I presume he's checking it with the engine at tickover in each case?

[georgemii]

You’re correct; my post was not clear.

What I meant is more like this:

Ignition will advance at higher RPM (due to counterweights), and
should return to original setting with engine at tickover.

I’m suggesting that the ignition is advancing when RPM increases,
but then fails to retard/return to original due to shaft or flyweights.

Does that sound better?

[sleeper bird]

i thought the advance was vacuumed operated? There are springs under the plate the petronix is mounted to?

[scumdog]

That’s right - there’s vacuum advance as you said but there’s mechanical advance, it is composed of two weights that swing outwards due to centrifugal force as the distributor shaft increases in speed as the revs increase.
These weights are under the plate that your points would have been attached to.
The weights are pulled back into their original place by springs.
As the poster above said - the weights for some reason are not returning to their original position, broken spring(s)? or the weights sticking on their pivot points?

As a footnote: if your timing sticks at 12 degrees advance you should notice the starter having to work harder to turn the motor over, possibly also cause the motor to ‘ping’ at low revs under load

[sleeper bird]

i understand now,can the dist be pulled without having to remove surge tank?

[sleeper bird]

also at what rpm,does/should the mechanical advance come into play?and how much advance does it provide im just curious.

[georgemii]

01. If we’re on the correct path here, this can be fixed without pulling the distributor.
Plate (where Petronix sits) is removed; you will then find the weights and springs.

02. In original configuration, the centrifugal advance is used for mid-range RPM.
This may have been changed - different weights and/or springs installed.

Let us know what you find….

[64ZCODE]

Yes. Once the cap is off and the Pertronix is removed, I believe there will be a base plate that will have to be removed to see the centrifugal advance mechanism. Should be able to tell then if the mechanism is sticking. Maybe some penetrating oil will be enough to free up the mechanism if stuck.

BTW, I run 12 degrees BTDC static advance on my '64 with a 390 with no pinging.

[sleeper bird]

i went ahead and pulled it so i could have a good look at it,i noticed that the springs connected to the weights were two different springs,one lighter than the other.When you move the weights they do return but not smoothly and i notice that the heavier spring actually has no tension when weights are returned in the park position(thats what i will call it)about a 32nd of play before tension on the weight starts.lube would help i suppose i did however order a rebuilt dist from napa figured at 68.00 why not.