The secondaries on the Holley 750 are a pair of throttle plates that activate in a delayed manner with respect to the primaries during brisk acceleration. The secondaries insert additional atomized fuel into the throttle bores, helping to smooth the transition from idle condition to full acceleration. Adjusting the secondaries includes tuning the throttle plate cam to advance or retard the opening of the secondaries and adjusting the size of the fuel delivery nozzles within the choke tower.
Unscrew the air filter wing nut, pull of the air filter cover, filter and filter base.
Locate the secondary throttle plate cam on the passenger side of the carburetor body. This cam has three holes, the top of which is the stock setting from the factory.
Use a set of pliers to remove the pin that holds the cam on the top setting. Push the plastic cam up with your finger until the next hole is aligned with the accelerator pumps cam pin hole.
Reinsert the pin to lock the cam into this elevated position.
Start the engine and operate the throttle body with your hand, while paying attention to the engine during acceleration. If any bogging is found, the cam will need to be adjusted higher again, to retard the opening of the secondaries. Bogging occurs when too much fuel is introduced into the engine, due to the secondaries opening too quickly. If bogging still occurs with the cam set to the highest setting, you'll need to reduce the size of the fuel delivery nozzle. These are located within the choke tower.
Open the choke tower flap with your hand and unscrew the screw head on the secondary throttle plate section. This screw secures the fuel delivery nozzles. Remove the screw from the choke tower.
Pull the nozzle out with a pair of needle nose pliers and note its stamped size number.
Insert a smaller nozzle using the needle nose pliers. Reinstall the screw and tighten.
Start the engine, operate the throttle body and ensure the bogging doesn't occur during acceleration.
