I'm guessing that he had effectively powered up the coil via the Tacho. The way the electronics works in a Tacho it would have basically:
A: Not have had full 12volts to coil
B: effectively shorted the B+ side of the coil to ground via the tacho electronics when he got off the throttle.
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Yeah I was thinking along the same lines except I would have spelt it dizzy.....
Bloody hell , im feeling dizzy ,dissy what ever after reading all that.:drink: