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3/29/17 12:20pm - Original Message: '427 3x2 Carb Vacuum Spring Question.'
Ole Biker Gold Member
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Monongahela, PA - USA

Vette(s):
1969 Corvette Coupe. 427/435 4 speed factory installed side pipes. Verified Numbers Matching.

Joined: 7/5/2016
Posts: 67
   Hello!
   I have the three two barrel carbs on my L71 engine.  the first and third carb are actuated by vacuum and controlled by the spring rate connected to the diaphragm in each.

   My question to anyone that has this same setup, what color spring are you using, satisfied with, on your engine.  I currently have the yellow springs in mine after testing heavier spring rates.  I am considering going to the next lower rate spring but want to get some input from others.

   Thanks For Looking,
   Mike C


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3/29/17 8:08pm - Reply: 'Re: 427 3x2 Carb Vacuum Spring Question.'
73shark
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Overland Park, KS - USA

Vette(s):
1973 Orange Metallic Coupe (orig owner), L82, 4 spd (WR), PS, (A/C & PW (I installed from wrecked 73)), leather, AM/FM Stereo, ran with '65 FI unit earlier & will again some day.

Joined: 7/9/2003
Posts: 397
I guess I'd try the same way that I use on my Q-jet and that go WOT from a rolling start.  Were any of the heavier springs stock?  If not, that's where I'd start.  That being said, I've never had the opportunity to work on a 3X2 BBC but a friend did own one back in the '70s and probably wishes he still had it.

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1973 L-82 4 spd

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4/1/17 5:06pm - Reply: 'Re: 427 3x2 Carb Vacuum Spring Question.'
Ole Biker Gold Member
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Monongahela, PA - USA

Vette(s):
1969 Corvette Coupe. 427/435 4 speed factory installed side pipes. Verified Numbers Matching.

Joined: 7/5/2016
Posts: 67
73shark said: I guess I'd try the same way that I use on my Q-jet and that go WOT from a rolling start.  Were any of the heavier springs stock?  If not, that's where I'd start.  That being said, I've never had the opportunity to work on a 3X2 BBC but a friend did own one back in the '70s and probably wishes he still had it.


   Hello,

   The carbs came factory with black coded springs that never opened fully.  I went to the plain colored spring, then to the yellow spring currently in the diaphragms.


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4/1/17 8:18pm - Reply: 'Re: 427 3x2 Carb Vacuum Spring Question.'
73shark
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Overland Park, KS - USA

Vette(s):
1973 Orange Metallic Coupe (orig owner), L82, 4 spd (WR), PS, (A/C & PW (I installed from wrecked 73)), leather, AM/FM Stereo, ran with '65 FI unit earlier & will again some day.

Joined: 7/9/2003
Posts: 397
How could you tell that the stock springs wouldn't allow the end carbs to fully open?

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1973 L-82 4 spd

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4/2/17 11:45am - Reply: 'Re: 427 3x2 Carb Vacuum Spring Question.'
Ole Biker Gold Member
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Monongahela, PA - USA

Vette(s):
1969 Corvette Coupe. 427/435 4 speed factory installed side pipes. Verified Numbers Matching.

Joined: 7/5/2016
Posts: 67
73shark said: How could you tell that the stock springs wouldn't allow the end carbs to fully open?


   Howdy,

   Basically seat of the pants feel from various cars I have owned or driven.  This monster should scare the Hell out of you when the two outer carbs kick in.  In addition, surfing the net has other commentaries that the black springs never allow full opening.

   My own experimenting verified, (seat of the pants), that each successive weaker spring gave better opening rates.  Currently the yellow springs, when opening, give the impression of a turbo kicking in.  My current thought is to have an even earlier opening rate.  Prior to changing springs again I was/am seeking other opinions on spring choice.  The car was put away for the winter so that eliminated further testing last year.  This year, while stored, I am rebuilding the entire rear suspension and differential so I have time to try and garner other opinions on the carb springs before getting back to that aspect of the car.

   With Regards,
   Mike C.






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4/2/17 4:57pm - Reply: 'Re: 427 3x2 Carb Vacuum Spring Question.'
plasticpig
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Vette(s):
1969, L71, K66, M21, C07, LeMans Blue (factory) currently Monaco Orange. 194679S70436

Joined: 9/6/2009
Posts: 3
As I understand it, the purpose of the spring is help in returning the center (primary) carb back to the idle position when you let off the gas pedal.
The Black spring would be for a 68-72 with a small block.
For the 69 Tri-Power the spring would be orange.
As long as your foot is heaver than what it takes to overcome the spring tension and the spring is weaker than the throttle cable, that part of the system will work.
The secondary's open with the vacuum diaphragms on each carb. (right side of the carb).  The function of the linkage is to prevent the secondary's (carb 1 & 3) from opening at a low RPM, (vacuum surge).  
Tried to post a photo of my spring which runs to the rear and is attached to the bracket that holds the throttle cable (mounts using the 2 bolts on the L side of the # 3 carb.) There is a small hole in my bracket. Yours may be correct with the little tab bracket on the #1 carb mounting bolt.  Either way it's just to return the throttle to idle.
I'd check your linkage between the front and rear to the "throttle closing lever", (little arm out the L side of the secondary carb that controls the throttle blades, which are the 2 round plates in the carburetor bores.
With the engine off, (air cleaner removed etc.) you should be able to twist the by hand the bell crank on the side of the center carb (better yet is to have some help who will push the accelerator to the floor to make sure the center carb is opening all the way)!    
Next is to adjust the linkage to the 1 & 3 carb., with the center carb in the idle (the engine is still off at this time!)
As memory serves twist the linkage to the #1 carb to where it just fits the "throttle closing lever" and then make one turn longer. Same with the rear carb.   
Make sure you have good smooth operation of the secondary linkage connections where they connect to the primary (#2 carb), good plastic bushings, proper bolt and nut etc.
I drive mine very little but at about 4000 RPM it is a real kick in the seat when they secondary's kick in.
Mike


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4/3/17 1:04am - Reply: 'Re: 427 3x2 Carb Vacuum Spring Question.'
73shark
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Overland Park, KS - USA

Vette(s):
1973 Orange Metallic Coupe (orig owner), L82, 4 spd (WR), PS, (A/C & PW (I installed from wrecked 73)), leather, AM/FM Stereo, ran with '65 FI unit earlier & will again some day.

Joined: 7/9/2003
Posts: 397
I thought the springs Mike was referring to were in the vacuum control similar to the one on a four barrel Holley with vacuum secondaries.

For butt dyno purposes you can continue to put a lighter spring in until you get a bog with WOT.

____________________________________

1973 L-82 4 spd

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4/3/17 11:26am - Reply: 'Re: 427 3x2 Carb Vacuum Spring Question.'
Ole Biker Gold Member
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Monongahela, PA - USA

Vette(s):
1969 Corvette Coupe. 427/435 4 speed factory installed side pipes. Verified Numbers Matching.

Joined: 7/5/2016
Posts: 67
plasticpig said:
As I understand it, the purpose of the spring is help in returning the center (primary) carb back to the idle position when you let off the gas pedal.
The Black spring would be for a 68-72 with a small block.
For the 69 Tri-Power the spring would be orange.
As long as your foot is heaver than what it takes to overcome the spring tension and the spring is weaker than the throttle cable, that part of the system will work.
The secondary's open with the vacuum diaphragms on each carb. (right side of the carb).  The function of the linkage is to prevent the secondary's (carb 1 & 3) from opening at a low RPM, (vacuum surge).  
Tried to post a photo of my spring which runs to the rear and is attached to the bracket that holds the throttle cable (mounts using the 2 bolts on the L side of the # 3 carb.) There is a small hole in my bracket. Yours may be correct with the little tab bracket on the #1 carb mounting bolt.  Either way it's just to return the throttle to idle.
I'd check your linkage between the front and rear to the "throttle closing lever", (little arm out the L side of the secondary carb that controls the throttle blades, which are the 2 round plates in the carburetor bores.
With the engine off, (air cleaner removed etc.) you should be able to twist the by hand the bell crank on the side of the center carb (better yet is to have some help who will push the accelerator to the floor to make sure the center carb is opening all the way)!    
Next is to adjust the linkage to the 1 & 3 carb., with the center carb in the idle (the engine is still off at this time!)
As memory serves twist the linkage to the #1 carb to where it just fits the "throttle closing lever" and then make one turn longer. Same with the rear carb.   
Make sure you have good smooth operation of the secondary linkage connections where they connect to the primary (#2 carb), good plastic bushings, proper bolt and nut etc.
I drive mine very little but at about 4000 RPM it is a real kick in the seat when they secondary's kick in.
Mike

Hello,

I have checked and performed those functions already.  I purchased a new linkage for the actuation of the outer carbs.  This package contained new nylon sleeves that act as bearings to smooth the movement of the linkage controlling the butterflies of the outer carbs. 

I also have the orange main carb return spring but the attachment point to manifold is not certain.  If you have a picture of just where the orange spring attaches to the intake manifold that would be a help!

Your statement regarding opening of the outer carbs at 4000 RPM is very helpful as that is where mine open currently. I suspect I will remove the yellow springs and install the weaker orange, I believe, weaker springs to see at what RPM they open and if there is a bog at WOT with them installed.

Do you, by any chance, know the color of the springs in your current setup?

With Regards,
Mike C.   



|UPDATED|4/3/2017 9:26:53 AM (AZT)|/UPDATED|


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4/3/17 11:22am - Reply: 'Re: 427 3x2 Carb Vacuum Spring Question.'
Ole Biker Gold Member
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Monongahela, PA - USA

Vette(s):
1969 Corvette Coupe. 427/435 4 speed factory installed side pipes. Verified Numbers Matching.

Joined: 7/5/2016
Posts: 67
73shark said: I thought the springs Mike was referring to were in the vacuum control similar to the one on a four barrel Holley with vacuum secondaries.

For butt dyno purposes you can continue to put a lighter spring in until you get a bog with WOT.


Hello!

Correct, my interest is with the opening of the outer carbs.

With Regards,
Mike C.


|UPDATED|4/3/2017 9:22:06 AM (AZT)|/UPDATED|


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4/3/17 7:49pm - Reply: 'Re: 427 3x2 Carb Vacuum Spring Question.'
plasticpig
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Vette(s):
1969, L71, K66, M21, C07, LeMans Blue (factory) currently Monaco Orange. 194679S70436

Joined: 9/6/2009
Posts: 3
Mike C., I have never had the vacuum pods opened on the carbs. I bought new replacements from the Chevrolet dealer in 1984 & had Holley rebuild and re-die before I took on the 50th Anaversity Caravan, (400 some miles of hot summer fun?). I'm out of town till Saturday evening, but will check the old dealer parts books I have, they usually give a good breakdown of little items like this. In the meantime, have you looked on the web, anything on Holley, not sure if they are reorganized? PS, Your engine looks great, I should have mentioned that in the first reply! Mike McElroy

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4/4/17 12:45pm - Reply: 'Re: 427 3x2 Carb Vacuum Spring Question.'
Ole Biker Gold Member
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Monongahela, PA - USA

Vette(s):
1969 Corvette Coupe. 427/435 4 speed factory installed side pipes. Verified Numbers Matching.

Joined: 7/5/2016
Posts: 67
plasticpig said: Mike C., I have never had the vacuum pods opened on the carbs. I bought new replacements from the Chevrolet dealer in 1984 & had Holley rebuild and re-die before I took on the 50th Anaversity Caravan, (400 some miles of hot summer fun?). I'm out of town till Saturday evening, but will check the old dealer parts books I have, they usually give a good breakdown of little items like this. In the meantime, have you looked on the web, anything on Holley, not sure if they are reorganized? PS, Your engine looks great, I should have mentioned that in the first reply! Mike McElroy


   Hello!

   Thanks for the compliment on the engine!  A bit of work on it and the carbs and the fuel lines to get it to look that way.

   The factory springs as came stock in the Vettes was black in color.  That is what I have read on various net articles.  My particular car on initial inspection had two different springs between the front and back diaphragms!!  Also, both rubber diaphragms were cracked and therefore leaked. 

   With Regards,
   Mike C


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11/21/17 8:23am - Reply: 'Re: 427 3x2 Carb Vacuum Spring Question.'
Ole Biker Gold Member
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Monongahela, PA - USA

Vette(s):
1969 Corvette Coupe. 427/435 4 speed factory installed side pipes. Verified Numbers Matching.

Joined: 7/5/2016
Posts: 67
plasticpig said:
As I understand it, the purpose of the spring is help in returning the center (primary) carb back to the idle position when you let off the gas pedal.
The Black spring would be for a 68-72 with a small block.
For the 69 Tri-Power the spring would be orange.
As long as your foot is heaver than what it takes to overcome the spring tension and the spring is weaker than the throttle cable, that part of the system will work.
The secondary's open with the vacuum diaphragms on each carb. (right side of the carb).  The function of the linkage is to prevent the secondary's (carb 1 & 3) from opening at a low RPM, (vacuum surge).  
Tried to post a photo of my spring which runs to the rear and is attached to the bracket that holds the throttle cable (mounts using the 2 bolts on the L side of the # 3 carb.) There is a small hole in my bracket. Yours may be correct with the little tab bracket on the #1 carb mounting bolt.  Either way it's just to return the throttle to idle.
I'd check your linkage between the front and rear to the "throttle closing lever", (little arm out the L side of the secondary carb that controls the throttle blades, which are the 2 round plates in the carburetor bores.
With the engine off, (air cleaner removed etc.) you should be able to twist the by hand the bell crank on the side of the center carb (better yet is to have some help who will push the accelerator to the floor to make sure the center carb is opening all the way)!    
Next is to adjust the linkage to the 1 & 3 carb., with the center carb in the idle (the engine is still off at this time!)
As memory serves twist the linkage to the #1 carb to where it just fits the "throttle closing lever" and then make one turn longer. Same with the rear carb.   
Make sure you have good smooth operation of the secondary linkage connections where they connect to the primary (#2 carb), good plastic bushings, proper bolt and nut etc.
I drive mine very little but at about 4000 RPM it is a real kick in the seat when they secondary's kick in.
Mike


   Hello Again!

   An update on the carb discussion posted earlier.

   I eventually weakened the vacuum pod springs on the outer carbs to a point where on the highway the outer butterflies were fluctuating open and causing very erratic cruising.  I went to a two step stiffer spring and this seems to have remedied the issue.  The current weather here cancelled any further testing.

   As far as the rod linkage adjust to outer carbs is concerned, I have FINALLY purchased the 1969 Factory Assembly Manual. The procedure for adjustment is as follows.  The rear carb linkage is attached to the center carb, then the linkage is extended till 1/2 diameter SHORT of the rod thickness, then attached to the rear carb throttle shaft.  this adjustment will allow the rear butterflies to just slightly open.  The linkage to the front carb is set at the center carb first, then extended till it reaches the CLOSED throttle shaft opening, then installed.

   Still a work in progress!

   With Regards,
   Mike C




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