Check ball or orifice? Jets?

If you have cold air [shaker] and headers / large or open exhaust, it will want richer jets, the hesitation with the 66's may be a hint. I always start super sewer-pipe rich and time it with a clock - then keep going leaner til it stops getting faster.

Sound like your secondary "flops" open no matter what. If you floor the primary by hand [engine off!] then push up on the secondary to open it, will it flop right open?
(Without crumpling the diagphragm I mean...)
If so, it maybe that somebody took out the check ball or ground out the orifice too large. That 'flopping' will be a problem no matter what spring you run.

I can "usually" feel it when the secondaries open but it is VERY subtle - if it's not, something is wrong, as you describe..