Bouncing Tacho

[Bumbledad]

Rev counter was bouncing and fluctuating, even at a steady speed - I have put a 1 meg ohm 10 turn variable resistor in line with the tacho feed, adjusted to get a trigger and now the needle is stable on a steady speed. But at 4000rpm the needle drops to zero, put the clutch in and it recovers - any ideas?

[blitzracing]

The spike that the coil generates drops in voltage as the revs rise and the dwell time drops, so you have simply caused the trigger voltage the tacho sees to drop to far. You need to get to the root cause- normally low resistance HT leads or poor earths

[Bumbledad]

Thanks, I'll try running a separate earth in and see if that helps....