Daimler V8 1968, Tachometer

Hello, My Tachometer sits idle or twitches slightly, a shot of petrol usually awakens it, but not always.
Is it a faulty genartor ?

An easy test would be to connect an AC voltmeter between the two terminals on the tach-generator and see what the voltage is there. It should read 10 Volts per 1000 RPMs, so at idle it should be about 7 Volts.

Just for my info: did the V8 Daimler saloons have the same type of tach generator system as the XK engined cars or was it a later style ignition driven system?

I think 1968 was about the time they changed from generator to ignition driven tachos, 420 used the latter and perhaps 240\340 and presumably V8 250. Need to know which type.