Motorcycle Repair: Shovelhead Tach, tach adapter, fire ignition

Question
Hi Mike,
I have a '73 FX made into an FXS.  It has the Low Rider speedo and tach on the tanks.  The tach is a replica of a stock tach ( 92042-78A ) of unknown brand ( maybe Drag, V-Twin, etc.)  The tach needle has always bounced but when I installed HI4 single fire ignition it got much worse.  I wired in a Crane tach adapter which cured the problem for a while but now the tach is all over the place again.  

Is there a way to see if the tach is bad?

Do I need a tach adapter with this ignition and stock type tach?  Crane could not answer that question.

Where do the three wires on the back of the tach go?  My schematic doesn't show the tach wires.

Thanks, Steve

Answer
If your tach is bouncing all over the place, its time to replace it. I dont know of anyone that repairs tachs. If you are using a single fire ignition then you will have to use a tach adapter on it or it will not read right. On the wiring, its hard to say what wire goes where since its aftermarket. They dont always use the same colors as stock.
Good luck and happy riding
Mike