Vacuum vs Mechanical Secondary’s?
02-13-2018, 08:19 PM
#1
Pro
Thread Starter
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.
02-13-2018, 08:46 PM
#2
Le Mans Master
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...
02-13-2018, 08:51 PM
#4
Le Mans Master
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.
02-13-2018, 08:59 PM
#5
Team Owner
Member Since: Jan 2007
Location: Southern Cal Ca
Posts: 50,465
Received 762 Likes
on
613 Posts
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?
02-13-2018, 11:04 PM
#6
Race Director
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.
02-14-2018, 12:16 AM
#7
Team Owner
Member Since: Jan 2007
Location: Southern Cal Ca
Posts: 50,465
Received 762 Likes
on
613 Posts
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.
02-14-2018, 12:32 AM
#8
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.
The following users liked this post:
Captain bob (02-16-2018)
02-14-2018, 03:13 PM
#9
Race Director
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.
02-14-2018, 04:06 PM
#10
Melting Slicks
[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?
02-14-2018, 04:55 PM
#11
Team Owner
Member Since: Oct 2004
Location: altered state
Posts: 81,242
Received 3,043 Likes
on
2,602 Posts
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.
02-14-2018, 06:21 PM
#12
Race Director
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...
02-14-2018, 06:47 PM
#13
Safety Car
Member Since: Apr 2001
Location: Sarver Pa
Posts: 4,569
Received 784 Likes
on
536 Posts
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; 02-14-2018 at 06:48 PM.
02-14-2018, 07:01 PM
#14
Race Director
02-14-2018, 07:26 PM
#15
[QUOTE=drwet;1596591195]Gopro and flashlight mount under the hood.
02-14-2018, 07:46 PM
#16
Le Mans Master
Member Since: Aug 2017
Location: Cool Northern Michigan
Posts: 6,896
Received 2,124 Likes
on
1,631 Posts
What does Bruce Springstein and a Double Pumper have in common?
Both are born in the USA and both Rock!
Both are born in the USA and both Rock!
Last edited by HeadsU.P.; 02-14-2018 at 07:48 PM.
The following users liked this post:
HeadsU.P. (02-15-2018)
02-15-2018, 12:52 AM
#19
Race Director
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.
The following users liked this post:
ajrothm (02-16-2018)
02-15-2018, 07:47 AM
#20
Race Director
Doesn't TBTR disagree with everybody about everything?