A tip for finding out if your Holley vacuum secondaries are actually opening.

I see a lot of threads on various forums where someone is trying to figure out if their Holley vacuum secondaries are really opening or not. Since they operate off I’d venturi vacuum and engine load trying to verify their operation is not really something that can be done in the driveway easily. This is one way to find out. This is on my ERA Cobra but all vacuum secondary Holleys operate basically the same way. Here is a zip tie fastened on the secondary throttle shaft and at rest, leaning forward with the secondaries closed. The zip tie needs to be pulled tight on the shaft so it has a friction fit.



At the primary throttle is opened up the bent link rod pulls forward in the slotted arm which lets the secondary throttle blades open up when venturing vacuum is high enough to activate the diaphragm pull on the other side that opens them. If after a drive and a good hard acceleration pull (probably 2nd or 3rd gear is best) the throttle shaft will rotate CCW inside the zip tie since it is resting on the carb body and cannot move. And when the throttle is released and the shaft will rotate back CW, the zip tie will now ride back with it in the CW direction and end up sticking approximately straight up if the secondaries opened fully. An easy tattletale to verify operation.


I would like to take credit for thinking this up but I learned it from a frustrated lawyer who would rather dabble with electrical circuits and mechanical gadgets. So if someone else has already posted this tip on here I wasn’t trying to plagiarize it.

That's why a double pumper leaves no doubt

Nice write up and thank you.

It is things like THESE that help people.
Thank you.

Thank you for that tip.

AR67GTX said:

I see a lot of threads on various forums where someone is trying to figure out if their Holley vacuum secondaries are really opening or not. Since they operate off I’d venturi vacuum and engine load trying to verify their operation is not really something that can be done in the driveway easily. This is one way to find out. This is on my ERA Cobra but all vacuum secondary Holleys operate basically the same way. Here is a zip tie fastened on the secondary throttle shaft and at rest, leaning forward with the secondaries closed. The zip tie needs to be pulled tight on the shaft so it has a friction fit.

View attachment 1895884

At the primary throttle is opened up the bent link rod pulls forward in the slotted arm which lets the secondary throttle blades open up when venturing vacuum is high enough to activate the diaphragm pull on the other side that opens them. If after a drive and a good hard acceleration pull (probably 2nd or 3rd gear is best) the throttle shaft will rotate CCW inside the zip tie since it is resting on the carb body and cannot move. And when the throttle is released and the shaft will rotate back CW, the zip tie will now ride back with it in the CW direction and end up sticking approximately straight up if the secondaries opened fully. An easy tattletale to verify operation.
View attachment 1895885

I would like to take credit for thinking this up but I learned it from a frustrated lawyer who would rather dabble with electrical circuits and mechanical gadgets. So if someone else has already posted this tip on here I wasn’t trying to plagiarize it.

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Couldn’t you put say a paper clip on the vac secondary diaphram shaft down low and drive and mash the pedal and see if the paper clip moved upwards after returning?

I don’t see why something like that wouldn’t work to as long as it would slide pretty easily but stay in place one the throttle is backed off.

AR67GTX said:

I don’t see why something like that wouldn’t work to as long as it would slide pretty easily but stay in place one the throttle is backed off.

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Something I read once and threw it out there

MWbirdLOVER70 said:

Couldn’t you put say a paper clip on the vac secondary diaphram shaft down low and drive and mash the pedal and see if the paper clip moved upwards after returning?

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I think it was Holley that published that paper clip trick umpteen years ago.

I knew when the secondaries weren't opening......but for the ones that are not familiar with the 'butt' feel, good write up!

I once read that Holley recommended that you shouldn’t be able to feel a big kick in the seat when you secondaries open. That the timing and transition of their opening should be seamless and fairly transparent as in just a stronger pull and if there was a big jump/kick then the timing was too late. But I figure, what’s the fun in that?

AR67GTX said:

I once read that Holley recommended that you shouldn’t be able to feel a big kick in the seat when you secondaries open. That the timing and transition of their opening should be seamless and fairly transparent as in just a stronger pull and if there was a big jump/kick then the timing was too late. But I figure, what’s the fun in that?

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Depends on how much power the engine has but even with a 289 with headers, Torker intake in a 66 Fastback I could tell when the 'the other half' wasn't opening. Someone mentioned earlier about using a DP carb and not worry about that happening....well, I agree lol.

AR67GTX said:

I once read that Holley recommended that you shouldn’t be able to feel a big kick in the seat when you secondaries open. That the timing and transition of their opening should be seamless and fairly transparent as in just a stronger pull and if there was a big jump/kick then the timing was too late. But I figure, what’s the fun in that?

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I’ve probably heard this 100 times. You really need to test it. Every VS carb I’ve tried would et much slower with the smooth opening secondaries on the street.

Years ago I took a piece of styrofoam from a cup, cut it to a size of a quarter, poked a small hole, then cut a slit in it. Fastened it the the diaphragm shaft (making sure it was snug) then drove the car. Worked like a charm. Hey, this should be my Shark Tank idea. I’m gonna be rich! It was nice knowing you all. Just kidding of course The zip tie idea is nice as well