Low vacuum pressure (3")

Discussion in 'Wrenchin' Secrets' started by Wauscion, Oct 24, 2019.

  1. Wauscion

    Wauscion Active Member

    Brand new disc conversion with booster and master etc.... vacuum pressure off the intake was 3" and car runs good and is smooth but may be a little rich. The braking is terrible and was the same way with drums. (Pb) pedal is rock hard at times. Sometimes they work great though. Electric vacuum pump needed? Thoughts or recommendations would be appreciated. 70 custom convertible. 350 2bbl roch.
     
  2. flynbuick

    flynbuick Super Moderator Staff Member

    With a stock cam the vacuum should be more than enough to handle the brakes. Perhaps a vacuum leak.

    If you went from front drums to disks you must have learned that the master cylinder has to be changed?
     
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  3. Mart

    Mart Gold level member

    A BIG one!
     
  4. Wauscion

    Wauscion Active Member

    With a big vacuum leak wouldn't the car have a rough idle or run ability issue? It has the hei distributor upgrade and also seems to have an issue when under heavy acceleration almost seems to backfire or popping noise. Only when kicks down. Wot
     
  5. Wauscion

    Wauscion Active Member

    Pardon my ignorance but can the engine be timed or manipulating the large vacuum leak?
     
  6. Mart

    Mart Gold level member

    Make certain all ports on manifold and carb are not hissing/ leaking. You should have over 15" vacuum.
     
  7. buick64203

    buick64203 Just plum crazy Staff Member

    You need at least 12 or 13 inches of vacuum for a soft brake pedal. 3" of vacuum seems hard to believe. You sure your gauge is working right? Does the car have a massive cam?

    You can do a quick and dirty test for a vacuum leak. While the engine is idling, slowing cover the top of the carb with your hand slowly smothering the air its taking in. As you do that, watch the gauge and take note of the rpm. If the engine has a vacuum leak, the rpm should go way up and the vacuum reading should go up as well.

    If there is no vacuum leak, the engine will die out because its getting choked
     
  8. flynbuick

    flynbuick Super Moderator Staff Member

    Yes—since the vacuum advance on the distributor would be disconnected from the vacuum tube and source during the process of timing.
     
  9. Wauscion

    Wauscion Active Member

    See photo of rear of carb
     

    Attached Files:

  10. buick64203

    buick64203 Just plum crazy Staff Member

    You hav an open port on there. Thats your leak. And that hose doesnt look good. Replace it and use a hose clamp
     
  11. Wauscion

    Wauscion Active Member

    Another
     

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  12. Wauscion

    Wauscion Active Member

    Stock cam 350 2bbl
     

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  13. Bens99gtp

    Bens99gtp Well-Known Member

    I have run very large cams for years, and even with very large cans with tons of overlap, and a low 800-900 rpm idle I always pulled about 5"a

    Is the gauge reading steady or very bouncy

    Something isn't adding up with smooth idle and very low vac
     
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  14. buick64203

    buick64203 Just plum crazy Staff Member

    He's got an open port at that rear fitting and that hose connection doesnt look very good either
     
  15. Bens99gtp

    Bens99gtp Well-Known Member

    Yep circled it in red . This needs plugged and the split end trimmed off
     

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  16. Wauscion

    Wauscion Active Member

    I will order a new pcv valve and 3/8 hose/clamp and a plug and see what happens. Thanks for the help. I will update this weekend.
     
  17. Wauscion

    Wauscion Active Member

    I am a greenhorn and appreciate all the input. I knew the nipple connection was not plugged but figured it was not necessary as the car runs well.
     
  18. Mart

    Mart Gold level member

    Check EVERY rubber hose you can find for dry rot , cracking or hard as a rock. Replace all with new.
     
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  19. Wauscion

    Wauscion Active Member

    This is the vac reading off of the uncapped nipple pictured above. I will try to plug it and see if it helps the braking.
     
  20. Wauscion

    Wauscion Active Member

    Sorry
     

    Attached Files:

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