Sometimes a failed winding in the stator. Sometimes a failed regulator/rectifier. Sometimes a bad Tach. Sometimes no 12v DC to the power post of the tach.
There is a DVA peak voltage number for the signal on the gray wire, coming off one of the yellow wires from the stator terminal buss, but I don't know it right off the top of my head.
The CDI Electronics website has troubleshooting guides for your motor's ignition components, find the stator for your motor, then click on the PDF file for instructions and TS guide.