Vacuum vs Mechanical Secondary’s?
Thread Starter
Pro
Joined: Feb 2006
Posts: 649
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From: Shelby Twp MI
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.
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.
Le Mans Master
Joined: May 2006
Posts: 9,993
Likes: 1,136
From: League City Tx
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...
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...
Le Mans Master
Joined: May 2006
Posts: 9,993
Likes: 1,136
From: League City Tx
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.
Team Owner
Joined: Jan 2007
Posts: 53,648
Likes: 878
From: Southern Cal Ca
St. Jude Donor '22
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?
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?
Race Director
Joined: Dec 2004
Posts: 10,332
Likes: 63
From: PARADISE
St. Jude Donor '09-'11-'12-'13-'14
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.
Team Owner
Joined: Jan 2007
Posts: 53,648
Likes: 878
From: Southern Cal Ca
St. Jude Donor '22
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.
Melting Slicks
Joined: Jun 2011
Posts: 3,116
Likes: 598
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.
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.
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Race Director
Joined: Apr 2000
Posts: 17,676
Likes: 201
From: Who says "Nothing is impossible" ? I've been doing nothing for years.
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.
You can use it on a mild 350CI to a wild 400ci monster, probably the most versatile carbs ever made.
Melting Slicks
Joined: Dec 2009
Posts: 3,233
Likes: 657
From: Thunder Bay
[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?
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?
Team Owner
Joined: Oct 2004
Posts: 81,241
Likes: 3,063
From: altered state
St. Jude Donor '05
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.
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.
Race Director
Joined: Dec 1999
Posts: 11,096
Likes: 990
From: Sierra Foothills CA
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...
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...
Le Mans Master
Joined: Apr 2001
Posts: 5,436
Likes: 1,253
From: Sarver Pa
2021 C1 of the Year Finalist - Modified
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.
Last edited by Pop Chevy; Feb 14, 2018 at 06:48 PM.
Melting Slicks
Joined: Jun 2011
Posts: 3,116
Likes: 598
[QUOTE=drwet;1596591195]Gopro and flashlight mount under the hood.
Race Director
Joined: Dec 1999
Posts: 11,096
Likes: 990
From: Sierra Foothills CA
You have to understand the history on this topic to know that TBTR and I went at it many times over this. He said that double pumpers were for race engines only that ran high RPMs only, and were totally unsuited for street use. AJ knows very well that I agree with him and pretty much everybody else here except TBTR. That's why I put the wink after my post to AJ. It's all good.
