I installed a used autometer procomp tach on a carbed 305. The first time i installed it, it worked perfectly. It would zero when power was applied to it with turning the key and would show my 850rpm idle and rev accurately.

Now the third time i started car up with it installed, about a week later, it wont zero out anymore and it rests at 1100 when turned off and idles at 2000rpms on the tach. What could cause this?