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
______________