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  • Question concerning Vacuum drop when under load.

    I have a question concerning the vacuum of my engine when at idle and in park it seems to keep a steady -18.4 to 18.6. I checked the timing and it is a little Erratic it moves Now when I have it in drive/under-load. it will drop to -15 and will decrease quickly to near -0.4 by 25 percent throttle and around -6 to -9 at 10 percent throttle. will probably make a youtube video with the phone recording the Edelbrock app digital display.
    Been Google Searching what can cause this some say loose timing chain. engine is near new with 7500 miles on it but i understand not every timing chain lasts like it should.

    My question for you guys are can my assumptions be right and it be the timing chain or can it be something else? what does this info tell you. and i like to repeat at idle in park it seems to run fine when warmed up. and i believe when cold has backfired into the manifold once or twice when throttling it while in park.

  • #2
    That's how vacuum works in a motor.

    The timing changes at idle. Because the system uses timing to regulate the idle speed. So. You will always see timing bouncing around at idle.
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    • #3
      Yes but when you setup initial timing you lock it in the settings and it is suppost to stay steady so you can set it to 12 btdc. And then again when I first installed the edelbrock pro flo 4 it did not seem the vacuum dropped off so quickly inside the manifold. It would be nice to see someone to record their data while driving and with slight throttle under 25% and again at 10% so can compare my reading it has not been running like it should actually fills as if something out of alignment whether it is the timing chain spark plugs cam or something else intierly.

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      • #4
        You want some one to "lock" their timing, and drive around, making a video?

        The instructions specifically say, don't do it.

        The motor will run bad after around 1200 RPM.
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        • #5
          if you have ever test driven a car with a vacuum gage you would see that under load vacuum will drop and under light acc it will very depending on a hill going down a hill and that is the way i have always seen timing on a computer controlled car will jump around as the inputs tell the e c u what is happening ad adjust as needed.

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          • #6
            No Bite_me I did not say for someone to lock their timing. I just want someone to record their edelbrock app so i can compare what vacuum is doing under load with light throttle.

            And yes Fleet man under load the vacuum drops but should it drop as quickly under load thats my question I have the edelbrock pro flo 4 installed on a 1975 chevy k20 SBC 350. I am just concerned that my engine has a issue that i cannot seem to figure out and would like info to compare to mine so i can check off some infomation. because if i am looking in the wrong area then i am wasting time.

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            • #7
              How to use a vacuum gauge for almost every situation: https://www.google.com/url?sa=t&rct=...xm8A8l8iJhNuaa
              At 43,861 MPH Elon Musk now owns the fastest car in this Galaxy

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              • #8
                depending on how much you press the gas peddle the vacuum will drop if you hammer it 0 vacuum will be seen if you went 25% throttle you might see 10 in of vacuum.

                back in the day they had fuel mileage meters witch were vacuum gauges that was colored to indicate good bad and high fuel mileage the e-zer you were on acc, to save fuel

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                • #9
                  This topic sucks.

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                  • #10
                    Fleet man thanks the problem is that it is at near 0 at 25% throttle and drops quickly with slight throttle. Thanks for the info.
                    also thank you bite_me for the PDF file. Will check next week but it is when I am driving it when the problem crops up also drove it today and it seems the longer I drive it the lower the vacuum gets when in drive and at a stop I think it was at -13.4 at one point at a stop light.
                    Last edited by Themoldyone; 10-18-2020, 09:42 PM.

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                    • #11

                      T in a manual vac guage to see if the MAP sensor is reading correctly.

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                      • #12
                        It's a Manifold "Absolute" Pressure ( MAP ) sensor. You have to be at Sea Level for them to match readings.
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                        • #13
                          Originally posted by Heatsoaked View Post
                          This topic sucks.
                          It's obvious that no one bothered to read page 5 of the PDF I linked to.
                          At 43,861 MPH Elon Musk now owns the fastest car in this Galaxy

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