[Jonny H]

I just realised you have an early car. In your '70, the tach is driven straight from the from the dizzy. This is the black and purple wire. It should connect at the distributor on a double spade. The other end goes to the 'speed sensor' (round silver Bosch can) then on to the tacho. I can't see any reason to have disconnected it when fitting the ignition.