Vacuum secondary carburetors operate based on the amount of vacuum, or draw that is "sensed" by the carburetor. Most vacuum secondary carburetors (QFT included) have a small hole in one of the primary venturi - this hole is extremely vital to the secondary's operation. As you increase the amount of throttle, the air speed in the venturi increases. As the air rushes past the aforementioned hole, "draw" or vacuum is produced. This draw will pull on the internal diaphragm of the vacuum secondary housing, causing it to open the secondary throttle shaft. Most vacuum secondary carburetors determine the rate of opening with different spring tensions - to change the rate you have to disassemble the housing and change the spring. A QFT carburetor is much more sophisticated, and simply requires a standard screwdriver to change the rate of opening. If you would like your secondaries to come in a little quicker, simply turn the screw counter-clockwise. This opens up an orifice to increase airflow and speed up the process. We recommend when turning the screw in either direction to turn no more than a 1/4 turn at a time until you are satisfied.

If the secondaries seem unresponsive or do not work, double-check that the orifice supplying draw is clear, and that the diaphragm is sealed properly and not torn.
The baseline setting on most of our carburetors is 1.5 turns off of the seat.