Tach hookup

I bought an HEI unit from TrailBeast and it is working great. Just now hooked up a Sun Super Tach II (I know, it's cheap but only for tuning, not racing) and when I set it on 8 CYL, I seem to only get half the rpm's. I am certain all the wires are correct. Am I missing something? When I put it on 4 CYL, it seems correct. Thanks. Dennis