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  1. #1
    FEP Power Member 85stanggt's Avatar
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    Default Need help with vacuum advance tube routing

    Driving the car more since fixing the head gasket. Runs strong! BUT, I realized there has been something wrong with the vacuum advance for years. I reset the idle and re-tuned the carb after the head job.

    Now, the car idles properly when cold, but after the engine warms up, idles at 1000-1200 rpm. When I pull the vacuum advance, the idle drops back down to 700rpm. I had apparently compensated for this by setting the idle to 700rpm while hot, which when cold would make the car barely idle at 500rpm.

    I have everything hooked up as per the underhood diagram: the vacuum line to the advance can goes to the triple connector on the vacuum tree on the intake that has the electrical plug on it. (Side note: about 15 years ago, I went through the vacuum system and the diagram to verify everything. The vacuum advance at that time was plugged onto the other vacuum tree on the intake without the electrical plug (the green or purple one). At the time, I switched them because the diagram clearly shows it going to a tree with an electrical plug.)

    So how are these vacuum lines supposed to be hooked up? How much vacuum at the advance should I be measuring at idle and how much advance should it contribute?

    Thank you!

    Edit: just had a thought...could I be getting too much vacuum to the advance because the primary throttle is adjusted open too far? Maybe I need to open the secondaries more to get the idle up?
    Last edited by 85stanggt; 10-10-2020 at 09:55 AM.
    1985 Mustang GT Convertible
    Stock and original @ 213k, except for dynomax ultraflos.

  2. #2
    FEP Super Member
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    Default

    You should be getting 0 vacuum at idle to the advance .
    clowns to the left of me , Jokers to the right

  3. #3
    FEP Power Member 85stanggt's Avatar
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    Default

    I think I answered my own question...I did some google searching and read through old threads. It appears that with the factory 4180c, the vacuum on the advance can varies based on engine temp due to the vacuum tree on the back of the intake.

    According to an old post from JACook, the distributor vacuum advance does indeed hook up to the vacuum tree with the electrical connection. That vacuum tree switches between the top and bottom vacuum ports at 130*F coolant temp.

    The center port goes to the vacuum advance. The bottom port gets manifold vacuum from the rear port on the intake. The top port gets "ported" vacuum from the front of the carb.

    When cold, the center port is open to the bottom port providing manifold vacuum to the vacuum advance. When the temp rises above 130*F, the vacuum tree switches the center port off the bottom line to the top line, "ported" vacuum from the carb. However, this "ported" vacuum sees a slight bit of vacuum at idle due to the design of the 4180c. So it is normal to get 5-7" of vacuum at idle at the advance can.

    In my case, I believe I turned up the idle when the engine was cold, due to it struggling to stay running. The increased primary throttle opening actually increased the ported vacuum seen at the hook-up, which when the engine was hot and the tree switched to ported, fed more vacuum to the advance. So I turned down the primary idle screw and increased the secondary idle stop screw to open the secondaries more to bump the idle. Then fine tuning of the idle is done by the main idle adjustment screw.

    I haven't been able to drive it, but it's idling normal now when fully hot and it's actually idling smoother. A test drive will tell for sure. The main reason for this goof up is due to Holley's instruction sheet included with the rebuild kits. The printing is a poor quality photocopy that seems to suggest 3-4 turns from contact on the secondary stop screw. This is way too much, so I went with the internet's 1/4 turn suggestion. I ended up turning the screw in about 1/2 turn more just now, so maybe the poor quality photocopy was meant to say 3/4 turn? Who knows.
    Last edited by 85stanggt; 10-10-2020 at 02:48 PM.
    1985 Mustang GT Convertible
    Stock and original @ 213k, except for dynomax ultraflos.

  4. #4
    FEP Senior Member burntorange84's Avatar
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    The two vacuum trees VCV ‘s are spark red/ orange, and green for thermactor and egr. The green opens around 130, the red is around 180 F. The temps are stamped on the hex usually. This is why running a 180 thermostat will cause idle and bogging problems if you don’t address this.

    If tuning the secondary stop screw, I use feeler gages to adjust the height, then when the idle mixture is all set, remove the carb once to adjust the screw for what thickness was needed. You have to insert the gages when the engine is off btw and get crafty to get them to stay put. Around a 1/4 turn was what I remember.

    -j
    _________________________________________
    1984.5 Mustang GT: org. 5.0, 5spd, 3.27's;
    GT-40's w/93 exhaust; t-bird TC brakes....

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