I just bought a 1985 CB650 Nighthawk and the tach is reading at about 12K RPMs when it's shut off. When running it moves but just to a near straight down position. It does move back and forth between the two positions as it is being ridden.

What happened and what do I need to do to fix it? To get it to start at "0" again?
I'm pretty mechanical, so .... I just can't find anything on this.