Mark,
In most cases, this is a wiring problem, or a problem with the magneto pickup. If you use the Search function on web page searching for "tach" you will find lots of messages about this. Here is an example.
http://www.337skymaster.com/messages...highlight=tach
One member suggested Ebay as a source for a replacement instrument.
There are also some messages about the indicator being faulty, obviously that could be the case for you.
I'm sure others will have suggestions as well.
Welcome to the site!
Kevin
webmaster