240zcv

The car has a Pertronix ignition. I am focused on the vacuum advance because this problem occurs during light to moderate acceleration and my understanding is that this is when ported vacuum from the front carb should be causing vacuum advance to occur. The throttle opening exposes the port to vacuum and manifold vacuum is not too low because the throttle is not wide open. The vacuum advance does not even start to move until the vacuum is higher than where the FSM says it should be at maximum advance. Everything in the distributor moves freely and I am testing it out of the distributor so I am reasonably sure it is not a mechanical problem in the distributor itself.

I am a new first time owner of a 72 240z. The car has a partial acceleration problem where the car will surge and stumble hard enough to chirp the tires in second gear. This only happens with partial throttle acceleration with rpm's below 3000. Flooring the accelerator causes the problem to go away. I have set the ignition timing at idle to 17 degrees BTDC. I have pulled and checked all of the spark plugs and their color indicates that the fuel mixture is correct. Compression is good and within 10% across all cylinders. I have balanced the carbs which were already very closely balanced. The spark advances with engine speed and the mechanical advance seems to operate freely. I have taken the vacuum advance off of the distributor and it appears to differ significantly from the specs in the FSM. My vacuum advance does not even begin to move until between 11 and 12 inches of vacuum. The specs in the FSM say that advance should begin at 4 inches of vacuum and be at maximum at just under 10 inches of vacuum. Could this be causing my partial acceleration problem? The vacuum diaphragm does not leak, it just appears that the spring is really strong. I would appreciate any suggestions on other things to look at for my acceleration problem and would like to know if anyone else has run into this issue with the vacuum advance before.