Vacuum jumping around at idle?
Le Mans Master
Joined: Aug 2017
Posts: 8,336
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From: Cool Northern Michigan
These Vac leaks are tough to find alright. Appears to be just one cylinder, although might be a skip part way thru the vid, indicating a second valve leak. Hard to say.
If you didn't glue both sides of your valve cover gasket, you'll have it made for valve inspection. I would pull one cover and look for a rocker that just refuses to loosen up a bit.
By jogging the IGN key, observe the number 1 Exh valve, checking for too tight of lash. Then #1 Int and right on down the line. Then do the other bank.
If you find one lashed too tight, just reset the lash when its partner valve is fully open.
Another slim & rare chance is a Intake gasket failed seal at one port only.
If you didn't glue both sides of your valve cover gasket, you'll have it made for valve inspection. I would pull one cover and look for a rocker that just refuses to loosen up a bit.
By jogging the IGN key, observe the number 1 Exh valve, checking for too tight of lash. Then #1 Int and right on down the line. Then do the other bank.
If you find one lashed too tight, just reset the lash when its partner valve is fully open.
Another slim & rare chance is a Intake gasket failed seal at one port only.
Race Director
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From: San Antonio, TX/Mahopac, NY
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No. If you raise the idle slowly, what happens to the gauge? Also, where are you connecting the vacuum gauge?
Thread Starter
Instructor
Joined: Jul 2017
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I'm connecting the gauge to the port that comes off the intake, all other vacuum things are disconnected and plugged up. I have a new gauge coming today to make sure it's not the gauge. I'll check later and see what happens when I raise the idle.
Race Director
Joined: Jul 2018
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From: San Antonio, TX/Mahopac, NY
2024 Corvette of the Year Finalist - Modified
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Tee the gauge into the unported vacuum advance port on the carb (assume your carb vacuum advance is connected here) and try it again, post another video if possible and if it still jumps around the same, show it while slowly raising the rpms.
Last edited by Factoid; Apr 5, 2021 at 11:33 AM.
Dr. Detroit
Joined: Mar 2012
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From: New Braunfels Texas
Jebby
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Race Director
Joined: Jul 2018
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From: San Antonio, TX/Mahopac, NY
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Fantastic! Well done! You must be relieved.
Melting Slicks
Joined: Dec 2009
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From: Thunder Bay
That’s not the same gauge. If you use a liquid filled gauge it will dampen the vibrations and give you a reading like your second video. A cheap gauge will bounce around like your first. Try it again with the first gauge. I suspect nothing has changed.
Thread Starter
Instructor
Joined: Jul 2017
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Pro
Joined: Aug 2000
Posts: 610
Likes: 167
From: Vancouver B.C.
OP, can you try that first vac gauge, that is from your mighty vac, on another performance type motor to see if it moves the same? I've never used my mighty vac as an engine test gauge, but there should be no reason why not. It's a vacuum gauge. Just wondering if it's just not able to handle the pulses from an engine, but rather to test and hold linear type vacuum like a vac advance can or headlight actuator,etc. First gauge suggests valve guides, second gauge suggests all pretty good.
Le Mans Master
Joined: Oct 2010
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From: melbourne florida
2023 C3 of the Year Finalist - Modified
I have my vacuum guage set up with a tee in the vacuum advance and always have because that's how the engine runs
Dr. Detroit
Joined: Mar 2012
Posts: 10,090
Likes: 4,015
From: New Braunfels Texas
If you take a reading from a runner with a big cam it will bounce like that.....going to the carb can even out all of the pulses in some cases depending on the style of intake.....
I always take vacuum readings from the carb as it is the closest to the throttle plate and representative of what the carb “sees”.....all 8 cylinders....not just one.
Jebby
I always take vacuum readings from the carb as it is the closest to the throttle plate and representative of what the carb “sees”.....all 8 cylinders....not just one.
Jebby
Pro
Joined: Aug 2000
Posts: 610
Likes: 167
From: Vancouver B.C.
If you take a reading from a runner with a big cam it will bounce like that.....going to the carb can even out all of the pulses in some cases depending on the style of intake.....
I always take vacuum readings from the carb as it is the closest to the throttle plate and representative of what the carb “sees”.....all 8 cylinders....not just one.
Jebby
I always take vacuum readings from the carb as it is the closest to the throttle plate and representative of what the carb “sees”.....all 8 cylinders....not just one.
Jebby