My BBK has a brass bolt/screw which is held in place by a small setscrew behind/under the part/cover that comes off. The brass bolt is hollow with a spring and a pin/piston sticking out of the bottom which opens the butterflies. I set the butterflies around a 1/16th" pushing/depressing the spring to do so. I then tightened the small setscrew. I also tapped the inside top to make spring changes to see what difference spring pressures do to response/driveability using a setscrew and nut to lock down.
.
Oh yes, there is a circular spring you have to mess with to get back on for the butterflies to snap closed! No biggie really, al least for me. I did this 2 years or so ago. I ended up reinstalling the original spring and played with the tension.
The throttle shaft was binding a little so removed it and straightened it out and used red locktite to reuse screws. The original blade were very free and this was Not, now is.