Incorrect tach reading?

I aquired an older (79) Deckette II deck boat with a Mercruiser 120. While on the water with the engine running at what sounds like a medium range (not wot) my tach is reading 6000 rpm, I know it's not running 6000 rpm. Also out of the water at idle the tach reads 2000 rpm with the engine bairly lopping along. How do I get the tach gauge to read correctly?

Thanks

Look on the back of the tach for a setting. It should be pointing to 4C.

Thanks, I'll check that out.

Well I forgot about this post. :) When I got a look at the back of the tach, it is busted. So I will be adding a new tach.