Secondary on carb not opening up?

84SN2001 wrote: can the secondary diaphram spring get weak over time?

I've never heard of this ever happening or being a problem.

There are two adjustments that can be done to control the opening rate on a vacuum secondary Holley, the diaphram spring and the kill bleed hole size.

The spring has more to do with opening speed.
The kill bleed hole has more to do with opening point.
Both have an effect on each other.

The spring adjustment is the most known and it's easily changed.

The kill bleed adjustment is rarely done, probably because most people aren't aware of the kill bleed. A larger kill bleed hole will raise opening rpm point by bleeding off vacuum available to pull on the diaphram. It's easy to make it larger, drill it.
A smaller bleed hole allows more vacuum to pull on the diaphram, thus lowering opening rpm point. Since a drill bit that drills a smaller hole hasn't been developed, the usuall mod to make the hole adjustable is to tap the bleed hole and install a small allen screw with a hole of the desired size drilled in it. Another way is to plug the hole with lead and redrill, but this isn't adjustable like a drilled allen screw. The quick and easy way to reduce hole size is to take a small piece of wire, bend it 90*, remove vac pod, stick one end of the wire into the hole, then reinstall vac pod.

BTW, the mechanical linkage closes the secondaries, not the secondary spring. If the secondary spring does become weaker, it's most likely from corrosion, not fatigue, and it will open the secondary a bit sooner and opening rate from closed to full open will be quicker.