The early 5200, the type Stovebolt sells, have a mechanical secondary. It will open at approx 70% throttle. If your's isn't opening, and you have mechanical unit, either your throttle or your secondary linkage needs to be adjusted. Easy to check throttle linkage but be sure to check pedal travel, i.e., don't assume pulling the throttle open from under the hood is the same as steping on the pedal. You can check the secondary at the same time. If you don't have the mechanical unit you probably have a later unit with the electronic fuel metering as well. This unit will work, but it isn't as good a choice as the mechanical carb.
Fun, and educational, these cars of ours.
Steve