Battery is good. I went out today to ride the bike and the tach worked for a few blocks and then quit again. So,is it possible the stator needs to be replaced? (if that's where the problem is)
The fact that it works then stops leads me towards a loose wire or ground. Check the wiring first then it may be best to pop in a stator if it's not too expensive. If you have a multi-meter, you can isolat the wires and check for proper voltage(and ground) in the wires leading to the Tach. Once you get hooked to the correct wires, you can rev the engine and expect to see changes in the voltage as ypou rev the engine. That would prove the Tach to be faulty. If the voltages stay constant or open(broken wire/bad stator), the tach is good. This is my best guess at how to help you diagnose but there are some actual Motorcycle technicians on this forum that would know better. I believe djmerullo is a MC Technician. Good luck.