If you are fitting an RVC tachometer where there was previously an RVI,

ignore the old signal wire and run a new wire from the coil / ecu / electronic ignition as appropriate

(if the old signal wire (usually white) connected to the tacho with a male and female bullet connector, connect them to each other.

When converting to an electronic ignition, add a new power feed from the ignition switch or fuse box.

This will minimise interference generated by the coil. If you power the electronic ignition or tachometer from the terminal on the coil, unpredicatable results (usually bad) may occur.

RVI Tachometer with Coil and Points

RVI Tachometer with Coil and Points

RVC tachometer with "Tacho Out" Electronic Ignition