This is an easy problem to diagnose and fix (and common). Take the small vacuum elbow hose going from the back of the carb to the vacuum actuator that controls the secondary butterfly shaft, attach a longer hose to that vac actuator, open the throttle by hand and apply suction with your mouth. If the secondary pulls open but won't hold vacuum - the diaphragm has split. Undo the retaining nut that hold the secondary vacuum actuator in place, remove the 'c' clip that holds the actuator rod on the linkage and you should be able to gently pry it out and replace it without removing the whole carb from the manifold. Easy fix