Mercury 800 '82 tachometer calibration?

[fishdog4449]

Hello All!
Got a great deal on a tach on ebay for my boat im restoring. It has a Mercury 800 on the transom; I followed the wire guidelines and hooked up the tach. Started her up, and it showed she was idling about 400 RPM. I know on some tachs you have to calibrate or set them for different engines, but I don't see any way to do that on this one. There is not selector or anything.
Any ideas?
heres a picture of the back of the tach

 

[Barnacle_Bill]

Re: Mercury 800 '82 tachometer calibration?

Please resize the picture. That tach appears to have been made for a specific application and may be the wrong one for your motor. And the tach itself maybe be bad as it has a lot of corrosion on the outside so I'm sure there is a lot on the inside.