Before I start it, tach is already around 500 RMP.

Gaffrig gauge, with a tattle tale switch and reset. But the tattle tale does not work.

Just bought the boat, so not sure how it should work (it looks simple and obvious, but you never know)

Motor is a 500 HP carb.

How can I check?

I hate the idea of buying a new tach, and nothing wrong with the old one.

Or maybe its that switch (not sure how that switch works)

Dont worry about it. Mine read "whatever" till I start it...then they read perfect. Yours is fine.

Tattle tail is for the GPS Speedometer. Should show top speed until reset. Gauge may be at 500rpm. When engine fires it should read correctly. Good luck

Mine do the same also

Thanks Guys,

Ya, mine starts @ 500, so looks broken. Glad you have the same reading.

It's a Gaffrid switch, with 3 postions. I think is says TACH, Recall & Erase.

Won't be at the boat till the weekend, will double check what they show.

O.K. Now I know something is wonky. THis weekend, was all over the place, or did nothing at all....

Where should I start to look to see where the problem could be?

THanks again!

Start at the back and work your way forward would be my advice.

Tachs often don't go to zero with the key in the off position. They should go to zero with the key in the on position and engine not running.

Thanks Griff, I'll check if its doing that. It does not read, or only shows 2000, its not close to being accurate.

New boat to me, so not 100% familar with the sound, but I think I'm bouncing off the rev limiter. Would like to try some props, but really need to get the tach in order before I start trying props.

Bring another tach down to the boat and run it in the motor compartment and check it with your dash tach. If its off start testing and checking connections and following wires.