Make absolutely sure the tach is actually GROUNDED as some columns are NOT well grounded. if it is mounted there. Make sure tach is powered from 12V switched and not from dimmer controlled lighting source or coil + !!!
It's not on the Column , none are metal all them are plastic housing . I have been wiring them under the hood so I can see what rpms I'm running trying to tune this carb and get my timing better .