You're right on the mark, it is for the Economizer Diaphragm, which operates the Power Valve.
Another thing that might cause the symptoms you describe, mainly the backfire on accelleration, is a lack of timing advance. Many times that is blamed on the carb, when it is actually an ignition issue. Check the vacuu advance for proper operation, as well as your base timing. You may need to advance it a few degrees.
