Vac. secondaries tuning - I want it all and right now!

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F-BIRD'88
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Re: Vac. secondaries tuning - I want it all and right now!

Post by F-BIRD'88 »

427dart wrote: Sun Jul 15, 2018 1:41 pm Now back in the day....1970 my brother had his new Hemicuda with of course the dual Carter 625 AFB's.
With the Torquflite auto and 4.10 Dana we would be cruising at 50 MPH and pull the shifter down to 2nd and hammer it at same time and there would a be a slight lag then a whomp of a power surge as you were pushed into the seat....hang on!!
Now you're talking'. 8)

You could run these on both your dual plane 2x4 intake AND on your good single plane racing intake (on a adapter).
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Re: Vac. secondaries tuning - I want it all and right now!

Post by 427dart »

F-BIRD'88 wrote: Sun Jul 15, 2018 1:36 pm I looked at the Eddy 650 AVS 2 carbs but didn't care for the different sized throttle bore/venturi from primary and secondary.

The running engine sees it differently.
Maybe maybe not. Each intake runner has it's own carb throat to pull on with a small connecting channel into other plane. Picture
four dots in a square pattern with a X connector at the bottom. This smaller cross over is for low speed cruising so you can run on the primary barrels only. It is not laid out like the Eddy dual quad RPM intake.
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Re: Vac. secondaries tuning - I want it all and right now!

Post by NormS »

Actually , in the same size venturi, the downleg boosters have a slightly weaker signal, making them tip in slightly later. But the overlap between the low speed circuits and high speed circuits is generally better with the downlegs. I used modified downlegs in that carb for the 355 street engine.
For double pumpers mounted end to end with real secondary metering blocks, I have used 4 primary side pivot bowls so that each carb could have fully functional accel pumps. If your carbs are too close together for real sec metering blocks, that presents the problem.
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F-BIRD'88
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Re: Vac. secondaries tuning - I want it all and right now!

Post by F-BIRD'88 »

Yup I know... It still won't mind that the primaries are different than the secondaries when running.

It is usually a net overall plus in actual performance.
OVERALL.
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Re: Vac. secondaries tuning - I want it all and right now!

Post by treyrags »

427dart wrote: Sun Jul 15, 2018 1:35 pm
treyrags wrote: Sun Jul 15, 2018 1:19 pm Do the carbs have the brass "signal tube" in the primary venturi that helps open the secobaries?
No these don't. Just the hole.
It is important for the actuation as well as the amount of opening
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Re: Vac. secondaries tuning - I want it all and right now!

Post by quickd100 »

I run 2 Carter 850 Thermoquads on a Weiand tunnelram on my 605 Hemi. I absolutely love those carbs.
[img]http://img.photobucket.com/albums/v514/quickd100/9ff3c690.jpg[/img]
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Re: Vac. secondaries tuning - I want it all and right now!

Post by rq375 »

3310 780 bodies have the provision for the secondary discharge nozzle (and down-leg boosters), but the feed holes need drilled. When the two holes are drilled, you end up with the feed hole right in the center of where the jet plate screws on. an 1/8" plate under the jet plate can carry a transfer slot from the float bowl accelerator pomp hole up to the hole in the body leading to the discharge nozzle. This ends up spacing the bowl out 1/8" but it looks like it will work on my intake, may need to remove the "ey" from the Holley logo on the opposed bowls. You will need two primary bowls, the fuel transfer tube holes will need plugged, and it will have fuel inlets on both sides.
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Re: Vac. secondaries tuning - I want it all and right now!

Post by pamotorman »

from the holley catalog.
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Re: Vac. secondaries tuning - I want it all and right now!

Post by jeff swisher »

Holley made a kit to make the secondaries mechanical on the vacuum carbs..It was sold by edelbrock.. many years ago.

We used to just stick a screw in the linkage. that will open them.
And we accelerated quicker and had more top end.

I had a 11+ compression 350 with 270H cam ported and milled 186 heads.
Ported Q jet intake and q jet carb..all dialed in that thing responded like a KZ 1000 when you hit the throttle.
I had the vacuum crap all pulled off the Q jet and the linkage was modified for mechanical secondaries.

I also lightened the air door so it opened really quick.. just enough tension to close it.
It liked it that way.
My uncle seen it and said WRONG!! that thing will be much quicker and responsive with the vacuum stuff and delayed opening set up correct.
Went on to tell me you can adjust the vacuum pots.

I told him have at it.. Here are the parts.
He was a NASCAR driver from way back ..raced against Petty and the others.
Well he spent about 2 hours messing with it and it just got worse and worse.. slow out of the hole not responsive under full mash of the go pedal.

He gave up and said freak engine like what it likes.
Never know until you try.
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Re: Vac. secondaries tuning - I want it all and right now!

Post by travis »

pamotorman wrote: Tue Jul 17, 2018 6:53 pm from the holley catalog.
I have never seen this before, but very interesting. Is this chart for a 750?
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Re: Vac. secondaries tuning - I want it all and right now!

Post by jmarkaudio »

treyrags wrote: Sun Jul 15, 2018 2:11 pm
427dart wrote: Sun Jul 15, 2018 1:35 pm
treyrags wrote: Sun Jul 15, 2018 1:19 pm Do the carbs have the brass "signal tube" in the primary venturi that helps open the secobaries?
No these don't. Just the hole.
It is important for the actuation as well as the amount of opening
This is very important on a 750/780 vacuum. Must be done. Info at link below.

https://www.tapatalk.com/groups/racingf ... -t205.html
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Re: Vac. secondaries tuning - I want it all and right now!

Post by travis »

.
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Re: Vac. secondaries tuning - I want it all and right now!

Post by pamotorman »

here you go
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Re: Vac. secondaries tuning - I want it all and right now!

Post by naukkis79 »

F-BIRD'88 wrote: Sun Jul 15, 2018 1:28 pm
Or better a pair of qjets run in a Six pack configuration.
(Front carbs primaries are not used, nore exposed to the plenum.)
4 big secondaries with central small responsive primaries.
Is somebody really used such a configuration? Qjet primary fuel metering is ok(well, for US standards), secondaries are just a bucket with holes on it. At stock configuration primary accelerator pump and primary venturi support quite a lot secondaries, and still it can't be dialed in perfectly - secondaries acceleration shot is from bathtub which has tiny fill holes - when you really drive car and don't just floor it mixture become way leaner than at first WOT.

So why use bad parts from two carbs and leave good parts unused? For low-vacuum accurate metering and emulsioning dual Qjet secondaries are probably worst configuration I could imagine.....
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Re: Vac. secondaries tuning - I want it all and right now!

Post by My427stang »

Although I haven't done it, I believe you can modify the carbs to have a brass signal tube into the primaries, you'll have figure out the spring after, but the more abrupt the edge of the tube, the stronger the signal
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