Okay, we finally got a chance to check some things.

First, compression. All cylinders measure between 145 and 155 PSI.

Second, checking the "roll over" point on # 6. We hand turned the crank (clockwise when looking from the front) until the intake on #6 started to open. Using a pointer as a reference, we marked this sopt on the crank pulley. Then we continued turning the crank until the exhaust valve closed and marked this spot. Turning the crank to the middle of these two marks showed the TDC mark on the dampener to be at 4* BTDC.

Would that cause the timing issues we're seeing?

Or do we have a vacuum leak or other problem?
Thanks,
Ken