390 vacuum advance ports - Vintage Mustang Forums
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post #1 of 8 (permalink) Old 08-20-2019, 11:54 AM Thread Starter
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390 vacuum advance ports

My 390 distributor has a vacuum advance with two ports. Here's a photo. The hose is connected to timed vacuum on my Edelbrock carb. Trying to figure out if it's correct. I think one of the ports on the vacuum advance is supposed to retard timing, but not sure of that. If so, I have no idea where to connect it.

Which vacuum advance port is which?

Full disclosure - this is on my 71 F100, not my Mustang. But I suspect the discussion could be applicable to a 390 Mustang.
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post #2 of 8 (permalink) Old 08-20-2019, 03:15 PM
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The blue cap is on the vacuum retard side, not the advance side. The system applies vacuum to both the advance and retard sides of the diaphragm. Someone capped the retard side. Take it off as the cap creates a sealed system and prevents the diaphragm from moving in either direction.

The Mustang vacuum routing can be found at the Mustang Barn site. https://www.mustangbarn.com/technical-information

I replaced my distributor with one for a 390 in an F150 with DuraSpark and an advance only diaphragm. You can get one at NPD, or RockAuto, or Summit. I had Dan at Mustang Barn re-curve it for my engine.
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post #3 of 8 (permalink) Old 08-20-2019, 10:34 PM
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The easiest thing to do is to replace the vacuum advance unit with a single port unit. The dual vacuum unit was used in conjunction with a DVCV and, potentially, a transmission modulator to control the vacuum advance curve. The simplest way to "solve" the dilemma when the fuel system has been modified is to get a new, single diaphragm, vacuum advance unit.
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post #4 of 8 (permalink) Old 08-21-2019, 09:04 AM Thread Starter
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Quote:
Originally Posted by Woodchuck View Post
The easiest thing to do is to replace the vacuum advance unit with a single port unit. The dual vacuum unit was used in conjunction with a DVCV and, potentially, a transmission modulator to control the vacuum advance curve. The simplest way to "solve" the dilemma when the fuel system has been modified is to get a new, single diaphragm, vacuum advance unit.
Is that a better approach than just removing the blue cap as CJM suggested?

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post #5 of 8 (permalink) Old 08-21-2019, 09:34 PM
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Quote:
Originally Posted by pollock View Post
Is that a better approach than just removing the blue cap as CJM suggested?
IMHO, yes. Simply uncapping the inner port will accomplish what you want, but will also leave that side of the diaphragm open to the elements and every time the front side "pulls" it'll pull in atmospheric air... you'll soon get corrosion, dirt and moisture inside.

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post #6 of 8 (permalink) Old 08-22-2019, 09:10 AM
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IMHO, yes. Simply uncapping the inner port will accomplish what you want, but will also leave that side of the diaphragm open to the elements and every time the front side "pulls" it'll pull in atmospheric air... you'll soon get corrosion, dirt and moisture inside.
Well, as much as I think Woodchuck really knows his stuff, even the single diaphragm unit has a breather on the side opposite the vacuum line or else that side would be sealed resulting in the same problem. Maybe I'm wrong, and the diaphragm is designed to fight a sealed side?
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post #7 of 8 (permalink) Old 08-22-2019, 09:32 AM
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Quote:
Originally Posted by CJM68GT390 View Post
Well, as much as I think Woodchuck really knows his stuff, even the single diaphragm unit has a breather on the side opposite the vacuum line or else that side would be sealed resulting in the same problem. Maybe I'm wrong, and the diaphragm is designed to fight a sealed side?
Nope, you're right. However, the opening is to the inside of the distributor where the cap provides protection from the elements.

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post #8 of 8 (permalink) Old 08-22-2019, 09:36 AM
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Which instantly reminded me of an incident that happened long ago with a friend who had a Jeep CJ7 with a 304 and a vacuum advance with a ruptured diaphragm AND a percolation problem. It seems that after shutting it off on day some fuel vapors made their way through the advance unit and into the distributor. Needless to say the next time he fired it up there was an unexpected "pop" as the distributor cap disintegrated. One of those weird occurrences you only run across once in a lifetime. Now, battery explosions....that's another story.

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