Tachometer anomaly

Regina and I are on Bribon, our '78 Cal 39 in Puerto Rico for a month. Tested engine as soon as we got here, seemed fine. Fired up to leave mooring and engine seemed to run normally but tach is reading 2000 at idle, instead of the usual high-ish 1000 rpm. Swapped out the voltage regulator with spare, same result. Tach is fed from the alternator anyway. Alt. is putting out 14.1V and charging ok, too. Any ideas? Please e-mail me directly with any suggestions, thanks. Peter "Sailboats are wind instruments!"