Dwell changes as vacuum advance works

[popbreed]

I have been chasing a miss in my 292 64f100 for several months. Miss occurs at speeds above 30 mph and is not constant. I checked over the distributor (rebuilt about 10,000 miles ago). Changed points (US made), changed the ground wire to a standard one (rebuilder had installed a heavier wire which made it not as flexible). Did not help. Rebuilt the carb because I noted the miss stops if you acclerate so I assumed this was because of the power valve opening and therefore I had a problem with the intermediate circuit. No help. Today I rechecked the ignition again and found the dwell decreased approx. 5 degrees as rpm's increased. Disconnectced and plugged the vacuum advance line and the dwell drop stopped. I now have the vacuum line caped and will test the PU on the road to see if that cures it. Question is, what would cause the dwell to drop as vacuum advance comes in? It would appear to me the rubbing block on the points is in the wrong position relative to the cam lobes. Theortically, as the breaker plate is rotated by the vacuum advance, the distance between the rubbing block and the cam surface must not chnage. Am I all wet? Anybody else seen this condition? Thanks for the help

POP

[Walston]

The dwell change seems normal to me. Have you set the timing with the engine warm and the vacuum advance plugged? Frankly it sounds to me like you have a bad plug wire or cracked distributor cap. You may try looking at the engine running in the dark. Race the engine and see if you have something sparking?