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