Basically the tach needle starts bouncing (like wiper blades) at around 7000 rpm and gets steadily worse. It will still pull 8000 but you can tell it's got a slight flutter. Basically the motor sounds thrashy. The motor doesn't quite pull as it should. Once the tach starts getting the weave I know the shaft is starting wobble. After I repair the distributor the tach goes back to normal.

Dave tell me if my logic is defective: the modified crank pulley has two blades to trigger the sensor, so it should trigger every 180 degrees which would be 90 degrees on the distributor, there is a plug wire every 90 degrees so it would in theory work (ignition box notwithstanding).