One thing I have learned is that with some MSD and or Pertronix conversions, if the tach does not read right away when you start the car, give the tach a good tap on the top. I've seen this work time and time again when people think there is something wrong and they think there is something wrong with the tach.

I think it has something to do with the old points system giving off an initial "jolt" of voltage that more or less jump started the old tachs whereas with the newer HEI conversions etc. you do not get that initial "jolt" as the newer systems are more regulated right from start up.

I hope that all makes sense and again, this is just my theory but it has proven true on a few occassions now. If this does happen, these tachs will start working once you start the car and drive for a few minutes or give the tach a "tap" and I bet it works. I know this is not how you want to get your tach working every time you drive your car, but I'm not sure how to correct that either...