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Carb Class: Vacuum Secondaries

Carb Class: Vacuum Secondaries

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 venturis - this hole is extremely vital to the secondary's operation. As you increase the amount of throttle air speed in the venturi increases. As the air rushes past the previously mentioned 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 Quick Fuel Technology 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 air flow and speed up the process. Slowing down the secondaries just as easy, just turn the screw clockwise to close off air flow. We recommend turning the screw in either direction 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 carburetors is 1.5 turns off of the seat.

If you need further assistance, please feel free to call us at 270-793-0900