[54blackhornet]

The two different tachometers are diagnostic types to use underneath the hood. Only two leads + red, - black. I connected the red to the HEI connection marked tach. Black directly to battery negative post. I really only need the tachometer to get everything dialed in properly after the HEI install and carburetor rebuild. Do I need to get a late model tachometer with more compatible with the HEI. The two I have used are very old, one from Sears, one from Wards…thanks for your help..Jack