Rev counter going nuts

[BRoadGhost]

As per title - it never used to but now it does; more often than not when up to temperature.

It actually doesn't do it when you're driving the thing hard, only when you're not...

The coil's brand new as is the ignition module.

Any ideas?

[Nova_Tek]

what engine?

[BRoadGhost]

C20XE on carbs with bosch ignition

[Nova_Tek]

Crank sensor maybe? Bad connection to the back of the clocks?

[Vausey]

mine also does this it has its moments

[BRoadGhost]

connections should be fine - it's as if sometimes the signal is too strong / irratic - I've already destroyed an extremely expensive tachometer because of this and now the less sensitive o.e item is doing the same.