Okay, my bad! Edelbrock carbs are designed based off of a Q-jet. After a little research for trying to help you they did a few things different. So it's NOT just a rebranded Q-jet, it just uses similar technology with a twist.
Just curious... How did you know the problem was when the secondaries open? I mean, how did you know the secondaries were opening? Did you hear them? Did you see them opening? Vacuum secondaries open much slower and smoother than mechanical secondaries.
If that was the case it is possible the metering springs/rods/jets are the culprit. As engine vacuum drops the secondaries slowly open and the metering rods lift allowing more fuel flow.
Try a Google search for "How to tune an Edelbrock carburetor".
I quickly found this -
Again, make sure the ignition and fuel delivery systems are functioning properly before tuning the carb.