I've been having a problem with acceleration on my '71 250C (what haven't I been having problems with!). The carbs have been rebuilt, no vacuum leaks, and tuned - idles perfectly with no RPM fluctuations. When I accelerate, I have to be very gentle on the pedal or else the engine chugs and surges and sometimes if I don't let-up, it will finally start to accelerate but with hesitations. It smoothes-out when I back-off the pedal and accelerate slowly again. I suspect the distributor vacuum advance? I looked at the 3-way valve and it clicks open/close when I remove the electrical lead, and am assuming that the lines are connected correctly based on the letters stamped by the connections: S=vacuum, F=advance, and U=retard? At idle, the RPM drops when I remove the advance line, so everything appears right. The timing does advance when I increase the throttle, but not much faster than when I increase the throttle with the advance line detached. With the engine off and I apply vacuum to the advance line, there isn't much resistance from the distributor, and the push rod moves just a little. Am I not applying enough vacuum or could the dist. diaphragm need to be replaced?