Carbs on these trucks are really pretty poor. From what you are saying it sounds like your carb is doing ok on the primary (up to 2500 rpm,) but when u push down on the throttle past that point and the secondary is supposed to kick in, it either doesnt or hesitates until more vacuum builds up and finally it opens.I am not a real pro on carbs, but I would start by checking for vacuum leaks in and around your carb.Be sure the four bolts holding the carb on the intake are nice and tight.Then I would get some carb cleaner and thorougly clean the carb,especially down the main body of the carb.Ur distributer works off a vacuum advance fromthe carb, so be sure that little vacuum hose isnt leaking as well and not pulling your dit advance around properly.Hope this helps.BJ