Likely cause of inaccurate tachometer?

So during the sea trials yesterday, one of the tachs would be like..way off for a while. At idle..would dance around 3500 or 4000. After a few minutes it would settle down and appear to be accurate. Would you guess an issue with the gauge itself, or sending unit?

It could be several things. There are some adjustments on the back of the tach that apparently can get crudded up (technical term). Some have had success turning those controls back and forth a few times and then of course returning them to their original positions.

I had an intermittent issue with my port tach that turned out to be a loose connection with the instrument harness on the back of the engine. Unplugging, cleaning the pins, and reconnecting that harness fixed it for me.

Whatever the case may be, a standard trick for trouble shooting such things is to swap parts betwixt port and starboard to see if the problem follows. In your case, if the problem follows your tach, the problem is the tach. If it does not, then it is a wiring or a sender issue.

Good idea about swapping sides....thing we don't think about when applying shade-tree mechanic automotive experience. (unless you have a multi engine car!)

I’m seconding the crudding up. Mine did very similar when I bought the boat. Unsteady needle and about 500rpm difference when the engines were synced. I pulled the panel and ran the indicator back and forth a couple times. Now it’s perfectly happy.

I thought mine would be a sensor, but I needed a new unit. With labor it costs about $325