The factory ca18 computer have a tacho wire. Ive seen computers where this wire no longer works after connecting to nonfactory tachos. But for most tachos, the factort wire just works.
For the 1200 style tacho you need the solder in 2 wires on the 12v to the coils. Each of these wires are wired basically parallel to the 12v coil feed.

Hope this helps.

hey guys random question
little bit stuck putting a tacho in my 620
as I've only ever wired them to carby engines/ dizzy
I have a layout of wires to the computer
which goes where but i dunno which one to use for the tacho..
thanks in advance
andy