The secondaries open up when your foot pushes too hard on the pedal next to
the foot brake. If you do not like that flushing sound of your hard earned
dollars being forced through the intake tract, lighten up on the pressure a
bit. Those secondaries make a holley four barrel double pumper seem like a
fuel saver by comparison. At road speed on flat country, they normally do
not open, but, anytime the vacuum dips below about 7" of water column, the
metering rods lift and the upper plate opens. Dumps fuel right out of the
float bowl through the main jets. $$$$$.