Tachometer Issues (Twin 7.4L)

[ertyqway]

Been having some really frustrating issues with my tachs recently, and am soliciting advice from the Maxum Brain Trust before I pull the gauges, sending units, and harnesses trying to fix it.

About one time in three now, when I start up one engine, the tach needle on the other (not yet started) will jump up and make it look like that engine is running too. More alarming, when both engines are running and the throttle levels are set equal, sometimes one of the tachs starts jumping around crazily or shows an rpm reading that is clearly not correct (last weekend, the starboard side jumped up to 6000 rpm and stayed there, even though I was only running at about 1300 in a no-wake zone). Scared the **** out of me when it happened, and this is why:

I am profoundly hearing impaired (i.e. deaf). So I can't really tell if the engine is running at crazy high rpm from sound alone. I am dependent on the gauges to know what the motors are doing. If the tach jumps to 6000 rpm, I have no way of knowing whether the engine is actually running that fast or not (apart from the fact that it would probably blow up at that speed).

This issue is intermittent. It doesn't always happen, or even happen a majority of the time. It occasionally switches sides (port-starboard), though it seems to happen more often with the starboard tach. After I've been running a bit, it will sometimes just settle down and "fix itself." Other times, It won't correct until I've shut both engines down and restarted them. Sometimes even that doesn't correct it.

Any idea what might be causing this?

[mmwjr]

Ok this sounds like a tach issue and the engineers are not really changing rpm.

Try rotating the cylinder select switch on the back of the tach back and forth a few times to see if that corrects the issue. Common problem and solution. If thus doesn't work either send them to the vendor for repair as they have limited life time warranty or just replace them.