Good Grief- You can adjust THAT!

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Joined
Jun 29, 2016
Messages
100
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Location
Phoenix AZ
My Car
Arizona car, 1973 Mach-1, Original 351 Cleveland engine and original C-6 trans, All original body panels, Interior is restored.
For every DIY-er who has an infuriatingly difficult-to-find problem in acceleration, I have found what the problem COULD be. I’ve had everything from my engine just flat-out die when I feathered the gas pedal to running fine with a little stagger that I just COULD NOT get rid of.

Timing was perfect. Gas line and filter were sterling clean. Cleaned and gapped plugs. Replaced plug wires. I replaced two pertronix units (and was ready to go back to points). Checked spark advance springs. Replaced advance dashpot (twice). All vacuum was good. Did all of this over a two-year period.

Then a friend says, “Did you adjust the advance dashpot?” WHAT!!

I didn’t know they HAD an adjustment!

Sure enough, I looked in the Autolite box, and there was a sheet where you look up your year and model and it tells you how many turns from “closed” to turn the 3/32nd Allen wrench you’ve stuck into the dashpot intake to make the proper adjustment for your car!

Now, some of the cheap ones aren’t adjustable, but the correct ones for our cars surely are, and now my car purrs and growls perfectly!
 
For every DIY-er who has an infuriatingly difficult-to-find problem in acceleration, I have found what the problem COULD be. I’ve had everything from my engine just flat-out die when I feathered the gas pedal to running fine with a little stagger that I just COULD NOT get rid of.

Timing was perfect. Gas line and filter were sterling clean. Cleaned and gapped plugs. Replaced plug wires. I replaced two pertronix units (and was ready to go back to points). Checked spark advance springs. Replaced advance dashpot (twice). All vacuum was good. Did all of this over a two-year period.

Then a friend says, “Did you adjust the advance dashpot?” WHAT!!

I didn’t know they HAD an adjustment!

Sure enough, I looked in the Autolite box, and there was a sheet where you look up your year and model and it tells you how many turns from “closed” to turn the 3/32nd Allen wrench you’ve stuck into the dashpot intake to make the proper adjustment for your car!

Now, some of the cheap ones aren’t adjustable, but the correct ones for our cars surely are, and now my car purrs and growls perfectly!
Sorry to see you spent so much time and money for such a simple fix. Glad you got your engine running fine.

We need to remember that there are two types of spark advance, mechanical and vacuum. Mechanical takes care of the first 32 to 36 degree and is based off of the engines rpm. The second is the vacuum advance and is added to the mechanical advance based off of the engines vacuum as you drive. Where to hook up the vacuum line is another topic where you will get a dozen different answer.

As a simple rule of thumb is that the vacuum line for a manual tranny should be hooked up to a ported source above throttle plates. This produces no advance at idle as since there is no vacuum at idle. The vacuum increases as the throttle is opened. For an automatic, going to a full-time vacuum source such as the manifold or the lower port on the carb will help to increase low-speed torque with the increased spark advance. As I stated, this is only a rule of thumb and can vary depending on the engine build and type of driving application.

The following is a post from Don C and chart he made with some ballpark numbers to help figure out exactly how much advance you are getting at any given vacuum. I also noted in there about how much advance you get per turn of the adjustment screw on adjustable pods.

https://7173mustangs.com/threads/vacuum-advance-adjustments.37381/
 
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I was under the impression that if the vacuum advance pot was hex shaped, that was the outwardly visible indication that you could stick a hex wrench into the hole to adjust it.

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