[Coley]

When I test a tach I (temporarily) wire it this way:
1) Grounded alligator clamp to the tin or tach casing (same thing). (use a good alligator clip or equivalent)
2) (-) terminal/spade & wire to the coil (-) connection
3) (+) terminal/spade & wire to the battery (+)

...either way, you will have to ground that tach housing or you won't get any action on the tach.

After that, I fire the engine and check the tach and ideally use a different separate tach to check the accuracy....which often isn't all that great compared to a digital tach.

All good
Coley