There are sometimes fine lines when tuning a carb . Raise the fuel level and the signal from the boosters does not need to be as strong to pull fuel so a rejet is sometimes needed.
I tweak and tweak on them..not as much as I used to as I can get really close with about 3 trips around the block.
One thing for sure is the engine needs to be warmed up as does the intake manifold .
Cold runners and engine and trying to test and tweak just do not go together.
Many times i have tuned on the highway. Took my jets and rods with me.
Long highway drive and pull over and change it and drive again and of course accelerate as quick as I can getting back on the highway
If you go Holley here are a few things I have done to them to make them pull harder on the top and mid range.
Depending on what air flow is needed i enlarge the venturies some and polish them.
I also thin the throttle shafts and either get small head screws for the throttle plates or machine the factory screw heads smaller.
I did this on 2 750's that would not pull as hard as my 750 AFB or my 750 Q jet.
One holley was a 3310 and the other a 4779 I also removed the choke tower.
I did not have any 800 or larger Holleys so it was not use them or try and make them flow a bit better and see if my top end improved.
After tweaking any of the 3 brands they all ran almost exactly the same.. to the point you could not tell i changed carbs.
I have never seen just a spring above the check ball in any of my AFB or edelbrocks..all mine had the brass puck and ball or a pointy puck and no ball.