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[C2] 3x2 secondary carburetor diaphragm vacuum

 
Old 07-11-2019, 05:58 AM
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ChristianGrebe
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Default 3x2 secondary carburetor diaphragm vacuum

Hi all,

I am wondering about the vacuum of the secondary carburetors in a Holley 3x2 setup.

When I operated a small vacuum hand pump I could not draw a vacuum at the nipple where the vacuum tube from the primary carburetor connects to the secondary. Sucking on the tube I could establish a vacuum large enough to operate the throttle blades. Clearly there is some passage in this vacuum system to the outside hidden in the secondary carburetor because you could hear air coming in somewhere when drawing a vacuum.

By inspecting the carburetor there seems to be a passage drilled from the vacuum connector in the direction of the air horn. Is this correct and does somebody know the use of this passage?

Thanks in advance,

Chris


3x2 secondary vacuum passage
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Old 07-11-2019, 07:39 AM
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tbarb
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Christian,

The vacuum operation of the end carburetors works exactly like the four barrel vacuum secondary carburetor.

Venturi vacuum that's created by air rushing into the primary carburetor creates a low pressure (vacuum) signal at the hole in the venturi and this signal is bleed off by the small hole in the secondary carburetor that you are hearing. This keeps the secondary carburetors from opening too soon. When the vacuum signal from the primary carburetor overcomes the secondary bleed hole the secondary carburetor opens up and at that point when air rushes in past the hole in the secondary the bleed hole changes from bleed to vacuum which aids the signal from the primary to completely open the blades.

Don't confuse venturi vacuum with manifold vacuum. The engine makes manifold vacuum under the throttle blades but It's the air rushing into the venturi that creates venturi vacuum, too completely different things..
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Old 07-11-2019, 08:11 AM
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DansYellow66
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OT - but wonder if anyone with a 3x2 setup has ever installed the Holley vacuum diaphragm caps that have a vacuum port on top and a piece vacuum hose to connect the front and rear carb vacuum units? It's quite common on Ford 2x4 setups to allow the secondaries on the front and rear carbs to open in sync and avoid a hunting back and forth if one opens first and then the second one comes in and causes the vacuum signal to decrease and the secondaries to flutter. If it works on the 2x4 vacuum secondary units I would think it would be beneficial on the 3x2 vacuum operated end carbs. Or is there another feature on the 3x2 setup that does the same thing to synchronize the end carbs opening?
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Old 07-11-2019, 01:39 PM
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Dan,

That's really the purpose of the secondary bleed/feed hole, to keep the vacuum servo from hunting.

Without it, if air rushes into the primary venturi causing the secondary to open and there is no bleed/vacuum feed hole in the secondary side, the air volume decreases on the primary side signal hole because of the flow through the secondary and then the secondary's close until air volume (primary side) is again enough to create the low pressure to open them again. Rinse/Repeat over and over again unless there is a secondary bleed/vacuum feed hole.
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Old 07-11-2019, 06:53 PM
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Got to thinking - I guess the outboard carbs get their secondary vacuum signal off the center carb - is that right? If so, then I guess the 3 carbs are all "Teed" together with a common vacuum signal so that synchronizes them.
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Old 07-11-2019, 07:00 PM
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426 Hemi
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Originally Posted by DansYellow66 View Post
OT - but wonder if anyone with a 3x2 setup has ever installed the Holley vacuum diaphragm caps that have a vacuum port on top and a piece vacuum hose to connect the front and rear carb vacuum units?
The front and rear pods are basically connected. Black hose with the tee:



1ASACE's pics.
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Old 07-11-2019, 10:11 PM
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99Silverbullet
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yes they are all connected, you can change how fast or how slow the end carbs open by changing the diaphram spring...or install a progressive linkage setup.
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Old 07-13-2019, 04:42 AM
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ChristianGrebe
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Thank you for your replies!

Can't wait, until my issues are sorted out and I can get those secondaries to work for the first time.
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