You probably need to tighten the nuts a little tighter on the Printed Curcuit Board that are for the tach. Two small nuts that are right next to each other on the back side of the tach. You do have the brown wire connected to the - coil terninal? To make the tach work you need these connections 12 volts + which is pin # 7 in gauge harness plug , brown wire to - coil terminal, which is pin # 9 and a grounded tach.