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Discussion Starter #1
Anybody else running the ZZ502 deluxe chevy crate motor, it comes with a 870 CFM Street Avenger Carburetor, electric choke and vacuum secondaries.


I noticed I wasn't getting much of a boost from the secondaries , so I did the paperclip test, checked for binding and then went to the lightest spring in the secondary kit.


the secondaries open 1/2 to 3/4 when wot and over 3500 rpm.


Car seems to run fine , but would like to have full use of the 4 barrel carb.


Holley told me that vacuum secondaries open up on demand, basically the motor isn't calling for more?


They also recommended a smaller carb.


So did Holley and Chevy miss-match the carb for this motor.


wondering if others are having the same experience?


Running a Tremec 6 speed T 56, 9 inch ford with 3.00 gear ratio.


I have also thought that possibly the gear ratio could be preventing the vacuum needed to pull the secondaries fully open.
 

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Discussion Starter #3
I do too, when I put my foot down I want everything to open.


I just hate to trash a new carb.


750 seems like a big drop down?
 

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More for Less Racer
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This thread on how the vacuum secondary signal is picked up in the primary barrel and fed to the vacuum pod, and how Holley has negatively impacted their carbs by leaving out a piece that used to be standard in all vacuum secondary Holleys.

https://www.tapatalk.com/groups/racingfuelsystems/vacuum-secondary-diaphragm-signal-tube-t205.html

Holley's Tech Departmentis never going to tell you the truth.

You may also want to search the other threads under a "vacuum secondary" search heading....there is some info on orifice and check ball just under the vacuum lid that may help you too.

RacefuelSystems forum has a wealth of Holley knowledge.
 

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The old 2 for one has worked for decades.....
500 cubic inches X 2 = 1000 CFM, depending on rpm's of course, but a 502 spinning up with a good size cam will want 1000. Heck, a good running 350 will go best with a 750, which is 2.14 times cubic inches.
 

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Discussion Starter #6
This thread on how the vacuum secondary signal is picked up in the primary barrel and fed to the vacuum pod, and how Holley has negatively impacted their carbs by leaving out a piece that used to be standard in all vacuum secondary Holleys.

https://www.tapatalk.com/groups/racingfuelsystems/vacuum-secondary-diaphragm-signal-tube-t205.html

Holley's Tech Departmentis never going to tell you the truth.

You may also want to search the other threads under a "vacuum secondary" search heading....there is some info on orifice and check ball just under the vacuum lid that may help you too.

RacefuelSystems forum has a wealth of Holley knowledge.
Thanks for sharing that link, it describes my carb issue exactly. The fix described looks pretty simple and inexpensive. I will follow up if I decide to install the tube.
 

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116 Posts
Discussion Starter #7
This thread on how the vacuum secondary signal is picked up in the primary barrel and fed to the vacuum pod, and how Holley has negatively impacted their carbs by leaving out a piece that used to be standard in all vacuum secondary Holleys.

https://www.tapatalk.com/groups/racingfuelsystems/vacuum-secondary-diaphragm-signal-tube-t205.html

Holley's Tech Departmentis never going to tell you the truth.

You may also want to search the other threads under a "vacuum secondary" search heading....there is some info on orifice and check ball just under the vacuum lid that may help you too.

RacefuelSystems forum has a wealth of Holley knowledge.

Thanks for sharing the article and forum, that seems to describe how my carb is performing exactly. I will update this thread if I decide to insert the tube. This fix looks simple to do and not costly at all.
The Holley tech seemed to be more interested in selling me another carb.
 
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