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

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.

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.

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

plasticpig said:

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.   

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.