Well folks, it looks like I can blame the stupid installer (me) for all my woes.
It turns out that Koso gauge is NOT fried and it was just shutting down to protect the internal circuits as there was a short in the speed sensor wiring. After tracing all the wiring circuits, I found the short; looks like the first go-kart run jostled the wiring and shorted out.
I rewrapped everything, plugged in the old Koso and it works!
I can now go back to see if the tach wiring is okay!! (likely as it was working before the test drive!).
Chock another problem up to incompetence.
Thanks to all who provided responses and tried to help me.