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Hello. If the tach is intermittent this is either loose connections,bad tach or failing rectifier/regulator.
We need to start with checking the connections on the back of the tach Make sure they wires are all tight. If good then we need to check the charging system. Check battery voltage before starting the engine, then check the voltage at the battery with engine running at 1500 rpms. Is the voltage greater when running than engine off? The maximum should be 14.5 volts. The minimum should be 13.3. Post back with findings.
Ok That's fine. I'll be here.
Yes. The engine should be connected to a standard cranking battery.
You need a group 24 and a minimum of 650 cold cranking amps.