Vacuum vs Mechanical Secondary’s?
I have a ‘77 with a fresh rebuilt 406 SBC - RPM Airgap intake, AFR 195 Heads and I just purchased a set of Hooker Super Comps to install. I also have a used (unknown history) Holley Street Avenger 770 w/vacuum secondary’s.
When I ran this last, just up and down the street, it seemed like a dud. The secondary’s did not seem to kick in, not sure if it was just some adjustment needed or do I even have the right carb? Since I have some downtime, should I look at rebuilding or perhaps an entirely new carb? Mechanical secondary’s? I’m not too sure of differences between the two besides the obvious. |
That's a pretty common problem with today's holley vacuum secondaries... It seems some of them just don't work that well, even with a lighter spring in the pod. I had brand new 3310 that the secondaries would only open about 1/2 on a 6500 rpm 396. It seems the port in the venturi that goes to the check ball in the vac sec circuit is just not big enough. I ended up having a 65' Holley 780 vac sec rebuilt, it has a little nozzle on the vac port in the venturi and the secondaries fully open on it.
Your combo will easily handle a 750 double pumper (4779) and that's what I would put on it. Then you KNOW the secondaries open, and it will hit much harder... Mechanical secondaries are really the way to go on a good/moderate street motor... |
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https://cimg9.ibsrv.net/gimg/www.cor...cf29ce3fa3.gif
This is the vacuum port for the vacuum secondaries, this seems to be the problem on the newer Holleys. Either its not drilled all the way through, or its not sized correctly. On the new carbs, its just a hole, no nozzle like this one in the pic. |
i haven't been able to verify my holley
secondary opening. tuning guides say you should not feel them open like a mechanical. how does one find out if and how much the sec open? |
Originally Posted by calwldlife
(Post 1596586195)
i haven't been able to verify my holley
secondary opening. tuning guides say you should not feel them open like a mechanical. how does one find out if and how much the sec open? |
Originally Posted by claysmoker
(Post 1596586888)
Put a paper clip on the rod that connects the diaphragm to the butterflies right at the base of the diaphragm can, then drive the car and hammer it. The clip will slide down the rod and stay at it's max opening point, and you can then see how much the butterflies moved when you're done. Super easy to do.
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Originally Posted by Emeightch
(Post 1596585940)
I have a ‘77 with a fresh rebuilt 406 SBC - RPM Airgap intake, AFR 195 Heads and I just purchased a set of Hooker Super Comps to install. I also have a used (unknown history) Holley Street Avenger 770 w/vacuum secondary’s.
When I ran this last, just up and down the street, it seemed like a dud. The secondary’s did not seem to kick in, not sure if it was just some adjustment needed or do I even have the right carb? Since I have some downtime, should I look at rebuilding or perhaps an entirely new carb? Mechanical secondary’s? I’m not too sure of differences between the two besides the obvious. Now if the diaphragm for the secondary's is leaking they won't work at all. This is my before and after repair. Also note they open as required. |
Just get the Holley 750cfm. Part #4779, they are quite inexpensive compared to other carbs and it works great right out of the box.
You can use it on a mild 350CI to a wild 400ci monster, probably the most versatile carbs ever made. :thumbs: |
[QUOTE=cagotzmann;1596587168]Ok you won't feel the secondaries kick in as they open as required.
Now if the diaphragm for the secondary's is leaking they won't work at all. This is my before and after repair. Also note they open as required. How did you get that video? |
OP have you made sure (or knows someone) your ignition curve is correct? That will make it feel like a totally different car! Id do that first then if it is still flat ck fuel pressure under load, then try a known good carb.
had that headache, was better after someone stuck the dist on a sun machine but still "missing something". Too the VS 750 off put a friends 650 DP on car felt like it was on nitrous, would boil the tires instead of just pulling then laying down Saved me a lot of frustration screwing with that thing (and was a new carb too boot) One thing at a time. |
Originally Posted by ajrothm
(Post 1596586119)
That's a pretty common problem with today's holley vacuum secondaries... It seems some of them just don't work that well, even with a lighter spring in the pod. I had brand new 3310 that the secondaries would only open about 1/2 on a 6500 rpm 396. It seems the port in the venturi that goes to the check ball in the vac sec circuit is just not big enough. I ended up having a 65' Holley 780 vac sec rebuilt, it has a little nozzle on the vac port in the venturi and the secondaries fully open on it.
Your combo will easily handle a 750 double pumper (4779) and that's what I would put on it. Then you KNOW the secondaries open, and it will hit much harder... Mechanical secondaries are really the way to go on a good/moderate street motor... |
Go find whoever you want. I agree with AJ. I went from a 750 vac secondary to a 750 double (along with a victor jr.)and the diff was night and day. The car will now spin the tires from a roll. That's on my 355.
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Originally Posted by Les
(Post 1596592242)
Well, I'm gonna hafta go find TBTR, or whatever he calls himself these days, to straighten your ass out! ;)
:rofl: |
[QUOTE=drwet;1596591195]
Originally Posted by cagotzmann
(Post 1596587168)
Ok you won't feel the secondaries kick in as they open as required.
Now if the diaphragm for the secondary's is leaking they won't work at all. This is my before and after repair. Also note they open as required. How did you get that video? |
What does Bruce Springstein and a Double Pumper have in common?
Both are born in the USA and both Rock! |
I also took my 670 Street Avenger off and replaced it with a 650 HP. It was a night and day difference.
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HeadsUP---That was gooood ! I'm a Springstein fan too.
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Originally Posted by Pop Chevy
(Post 1596592443)
Go find whoever you want. I agree with AJ. I went from a 750 vac secondary to a 750 double (along with a victor jr.)and the diff was night and day. The car will now spin the tires from a roll. That's on my 355.
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Doesn't TBTR disagree with everybody about everything?
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