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  •  » Erratic low vacuum reading at 12-15Hg help? (video) 302 on a 1965

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    7/07/2021 1:26 PM  #1

    mustang6518
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    Registered: 2/15/2021
    Posts: 31

    Erratic low vacuum reading at 12-15Hg help? (video) 302 on a 1965

    Anyone here good with vacuum readings to determine engine problems / vacuum leaks? Running a 20+ year old custom built 302 with unknown cam on a 65 Mustang and I'm getting a fast erratic 12-15 Hg vacuum reading (short video here). If I throttle up and hold it at 2000RPM, I get a slightly more steady 19-20 Hg reading (short video here). Any ideas what issue these readings indicate?

    I tried spraying around some carb cleaner to see if idle picks up due to a vacuum leak and it didn't seem to do anything.

    Reason I decided to check was I installed the Mustang Steve power brake pedal and booster a few months back (to replace a CSRP booster and stock pedal) and still haven't been able to get enough stopping power. Have gone through a huge amount of brake troubleshooting, and all that's really left seems to be this potential vacuum issue. Was considering adding in an electric vacuum pump, but want to make sure I've exhausted all other options first. 

    Last edited by mustang6518 (7/07/2021 4:59 PM)

     

    7/07/2021 8:32 PM  #2

    Nos681
    Senior 24-7 FYI Addict

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    From: Atlantic Mine, Michigan
    Registered: 7/14/2016
    Posts: 3,333

    Re: Erratic low vacuum reading at 12-15Hg help? (video) 302 on a 1965

    While troubleshooting, disconnect and cap/plug line to vacuum booster.

    See what kind of results you get.

    What cam and initial timing?
    Might try different initial to see if it changes.

     

    7/07/2021 9:45 PM  #3

    6sally6
    Hyper FYIFORD ADDICT

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    From: Surfside Beach,SC
    Registered: 4/02/2013
    Posts: 6,853

    Re: Erratic low vacuum reading at 12-15Hg help? (video) 302 on a 1965

    Give it more timing..........
    adjust carb (air mixture screws)to maintain highest vacuum reading
    adjust idle down to 750RPM or so and make another video!
    6sal6

    Get busy Liv'in or get busy Die'n....Host of the 2020 Bash at the Beach/The only Bash that got cancelled  )8
     

    7/08/2021 8:54 AM  #4

    BobE
    Senior 24-7 FYI Addict

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    From: White Plains, NY
    Registered: 4/04/2013
    Posts: 2,645

    Re: Erratic low vacuum reading at 12-15Hg help? (video) 302 on a 1965

    Regarding the vacuum, has this erratic condition always been there, or is it a recent issue?
    Did you check for leaks around the intake manifold to cylinder heads, carb base, PCV hose connections, (and try plugging the PVC valve to see if the condition improves), vacuum hose to distributor?

    It would be helpful to provide details of your brake system; drum or discs or combination (including the sizes), master cylinder type (single or dual bowl, bore diameter) and any other changes made to the original system. 

    65 Fastback, 351W, 5-speed, 4 wheel discs, 9" rear,  R&C Front End.
     

     

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