Vacuum vs Mechanical Secondary’s? - CorvetteForum - Chevrolet Corvette Forum Discussion


C3 Tech/Performance V8 Technical Info, Internal Engine, External Engine, Basic Tech and Maintenance for the C3 Corvette

Vacuum vs Mechanical Secondary’s?

Reply
 
 
 
Thread Tools Search this Thread
Old 02-13-2018, 09:19 PM   #1
Emeightch
CF Senior Member
Thread Starter
 
Member Since: Feb 2006
Location: Shelby Twp MI
Posts: 523
Thanked 1 Time in 1 Post
Default 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.
Emeightch is offline   Reply With Quote
Old 02-13-2018, 09:46 PM   #2
ajrothm
CF Senior Member
 
ajrothm's Avatar
 
Member Since: May 2006
Location: League City Tx
Posts: 8,574
Thanked 444 Times in 334 Posts
Default

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...
ajrothm is offline   Reply With Quote
Old 02-13-2018, 09:49 PM   #3
ajrothm
CF Senior Member
 
ajrothm's Avatar
 
Member Since: May 2006
Location: League City Tx
Posts: 8,574
Thanked 444 Times in 334 Posts
Default

ajrothm is offline   Reply With Quote
Old 02-13-2018, 09:51 PM   #4
ajrothm
CF Senior Member
 
ajrothm's Avatar
 
Member Since: May 2006
Location: League City Tx
Posts: 8,574
Thanked 444 Times in 334 Posts
Default





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.
ajrothm is offline   Reply With Quote
Old 02-13-2018, 09:59 PM   #5
calwldlife
CF Senior Member
 
calwldlife's Avatar
 
Member Since: Jan 2007
Location: Southern Cal Ca
Posts: 37,916
Thanked 27 Times in 27 Posts
Default

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?
calwldlife is offline   Reply With Quote
Old 02-14-2018, 12:04 AM   #6
claysmoker
CF Senior Member
 
claysmoker's Avatar
 
Member Since: Dec 2004
Location: PARADISE
Posts: 8,599
Thanked 46 Times in 42 Posts
St. Jude Donor '09-'11-'12-'13-'14
Default

Quote:
Originally Posted by calwldlife View Post
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?
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.
claysmoker is offline   Reply With Quote
Old 02-14-2018, 01:16 AM   #7
calwldlife
CF Senior Member
 
calwldlife's Avatar
 
Member Since: Jan 2007
Location: Southern Cal Ca
Posts: 37,916
Thanked 27 Times in 27 Posts
Default

Quote:
Originally Posted by claysmoker View Post
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.
calwldlife is offline   Reply With Quote
Old 02-14-2018, 01:32 AM   #8
cagotzmann
CF Senior Member
 
cagotzmann's Avatar
 
Member Since: Jun 2011
Location: Sherwood Park Alberta
Posts: 1,837
Thanked 118 Times in 99 Posts
Default

Quote:
Originally Posted by Emeightch View Post
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.
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.

cagotzmann is offline   Reply With Quote
The Following User Says Thank You to cagotzmann For This Useful Post:
Captain bob (02-16-2018)
Old 02-14-2018, 04:13 PM   #9
MotorHead
CF Senior Member
 
MotorHead's Avatar
 
Member Since: Apr 2000
Location: My other ride is your girlfriend
Posts: 17,476
Thanked 106 Times in 91 Posts
Default

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.
MotorHead is offline   Reply With Quote
Old 02-14-2018, 05:06 PM   #10
drwet
CF Senior Member
 
drwet's Avatar
 
Member Since: Dec 2009
Location: Thunder Bay
Posts: 1,639
Thanked 64 Times in 64 Posts
Default

[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?
drwet is offline   Reply With Quote
Old 02-14-2018, 05:55 PM   #11
cuisinartvette
CF Senior Member
 
Member Since: Oct 2004
Location: Valencia Ca.
Posts: 64,193
Thanked 752 Times in 692 Posts
St. Jude Donor '05
Default

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.
cuisinartvette is online now   Reply With Quote
Old 02-14-2018, 07:21 PM   #12
Les
CF Senior Member
 
Les's Avatar
 
Member Since: Dec 1999
Location: Sierra Foothills CA
Posts: 8,075
Thanked 349 Times in 246 Posts
Default

Quote:
Originally Posted by ajrothm View Post
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...
Well, I'm gonna hafta go find TBTR, or whatever he calls himself these days, to straighten your *** out!
Les is offline   Reply With Quote
Old 02-14-2018, 07:47 PM   #13
Pop Chevy
CF Senior Member
 
Pop Chevy's Avatar
 
Member Since: Apr 2001
Location: Sarver Pa
Posts: 2,738
Thanked 120 Times in 109 Posts
Default

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 07:48 PM.
Pop Chevy is offline   Reply With Quote
Old 02-14-2018, 08:01 PM   #14
claysmoker
CF Senior Member
 
claysmoker's Avatar
 
Member Since: Dec 2004
Location: PARADISE
Posts: 8,599
Thanked 46 Times in 42 Posts
St. Jude Donor '09-'11-'12-'13-'14
Default

Quote:
Originally Posted by Les View Post
Well, I'm gonna hafta go find TBTR, or whatever he calls himself these days, to straighten your *** out!

claysmoker is offline   Reply With Quote
Old 02-14-2018, 08:26 PM   #15
cagotzmann
CF Senior Member
 
cagotzmann's Avatar
 
Member Since: Jun 2011
Location: Sherwood Park Alberta
Posts: 1,837
Thanked 118 Times in 99 Posts
Default

[QUOTE=drwet;1596591195]
Quote:
Originally Posted by cagotzmann View Post
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?
Gopro and flashlight mount under the hood.
cagotzmann is offline   Reply With Quote
Old 02-14-2018, 08:46 PM   #16
HeadsU.P.
CF Senior Member
 
HeadsU.P.'s Avatar
 
Member Since: Aug 2017
Location: Cool Northern Michigan
Posts: 775
Thanked 111 Times in 106 Posts
Default

What does Bruce Springstein and a Double Pumper have in common?

Both are born in the USA and both Rock!

Last edited by HeadsU.P.; 02-14-2018 at 08:48 PM.
HeadsU.P. is offline   Reply With Quote
Old 02-14-2018, 09:17 PM   #17
Crimson Thunder
CF Senior Member
 
Member Since: Jun 2015
Location: Doctors Inlet Florida
Posts: 284
Thanked 31 Times in 29 Posts
Default

I also took my 670 Street Avenger off and replaced it with a 650 HP. It was a night and day difference.
Crimson Thunder is online now   Reply With Quote
Old 02-14-2018, 11:20 PM   #18
Pop Chevy
CF Senior Member
 
Pop Chevy's Avatar
 
Member Since: Apr 2001
Location: Sarver Pa
Posts: 2,738
Thanked 120 Times in 109 Posts
Default

HeadsUP---That was gooood ! I'm a Springstein fan too.
Pop Chevy is offline   Reply With Quote
The Following User Says Thank You to Pop Chevy For This Useful Post:
HeadsU.P. (02-15-2018)
Old 02-15-2018, 01:52 AM   #19
Les
CF Senior Member
 
Les's Avatar
 
Member Since: Dec 1999
Location: Sierra Foothills CA
Posts: 8,075
Thanked 349 Times in 246 Posts
Default

Quote:
Originally Posted by Pop Chevy View Post
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.
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.
Les is offline   Reply With Quote
The Following User Says Thank You to Les For This Useful Post:
ajrothm (02-16-2018)
Old 02-15-2018, 08:47 AM   #20
derekderek
CF Senior Member
 
Member Since: Jul 2015
Location: Westville NJ
Posts: 1,757
Thanked 154 Times in 140 Posts
Default

Doesn't TBTR disagree with everybody about everything?
derekderek is online now   Reply With Quote
Go Back   CorvetteForum - Chevrolet Corvette Forum Discussion >
Reload this Page
  • Vacuum vs Mechanical Secondary’s?
  •  
     
    Reply

    Related Topics
    Thread Thread Starter Forum Replies Last Post
    Tell me about 327/350 carb substitutes 68hemi C1 & C2 Corvettes 23 02-10-2018 11:51 PM
    Which carb to use for 489ci? Günther-C3 C3 Tech/Performance 39 09-21-2017 10:14 PM
    WTB Used Holley 670 Ultra Avenger Street Rat C3 Parts for Sale/Wanted 4 01-26-2017 10:11 AM
    1967 400/hp 4sp tri- power problems moonframer C1 & C2 Corvettes 40 09-20-2016 07:02 PM
    Carb jetting primary vs secondary cagotzmann C3 Tech/Performance 19 09-13-2016 03:57 PM


    Thread Tools Search this Thread
    Search this Thread:

    Click for Advanced Search

    Posting Rules
    You may not post new threads
    You may not post replies
    You may not post attachments
    You may not edit your posts

    BB code is On
    Smilies are On
    [IMG] code is On
    HTML code is Off
    Trackbacks are Off
    Pingbacks are Off
    Refbacks are Off

    Forum Jump

    Sponsored Ads
    Vendor Directory

    All times are GMT -4. The time now is 06:04 PM.


    We are a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for us to earn fees by linking to Amazon.com and affiliated sites.
     
    • Ask a Question
      Get answers from community experts
    Question Title:
    Description:
    Your question will be posted in: