I use a tach for diagnostic purposes only. Here is the one I use:
- ENM tach Model PT15B2 (includes hour meter)
http://www.enmco.comPicture:
I've got the wire wrapped around the spark plug wire, with the free end tucked away.
When I need to set the idle speed, I just hook the tach up to the wire and set the idle speed. When I'm done, I disconnect the wire, tuck it back away, and put the tach back in my drawer.
IHTH!