I have had my boat out twice since I bought it in February , when I first took it out I noticed that both tachs fluctuated 500 rpm when running and both gas gauges did to. I took it out weekend before last and done the same thing this time the starboard tach was reading 500 rpm higher and it was still bouncing like the port along with the gas gauges. I do have one battery not holding a charge but that shouldnt effect both tachs since they are isolated power wise from each other. Any thoughts, common ground maybe?

Ground

Bad plug wires

Wouldn't bad plug wires cause the engine to miss? When it does this both engines sound in synch with one another and no sign of miss fire?

You've most likely got a ground short. Check the grounds at the fuel tank sending unit.

No you do not have to have a miss. A good friend with a 25 Eliminator fought this problem for days and finally changed the plug wires and it went away instantly. He told me about it when I had a similar problema nd I changed the wires and all was good and I did not have a miss at all. Sorry I do not know why or how to explain it. Just a suggestion based on experience.

Did those erotic gauges cost extra??? Just kidding, the battery will cause some crazy sh*t. Replace that before you go looking for trouble.

Ditto start with the battery, check the grounds and then go for the plug wires.

Could you explain this, cuz if it was shorted to a positive leg it would blow a fuse or heat up and burn the insulation.

Kenny, bite me but I did get two new battery's today and will try it out this weekend.

Doc, thanks im going to try this using a set of known good wires.

I have been chasing the same problem for a couple of seasons checked all grounds cleaned everything i could see finally sent all gauges to livorsi this winter they rebuilt several of them i hope this fixes if not i will try plug wires

Hey Trips I have had the same problem the last two seasons at or around 3500-4000 rpm's. Please let me know if you have corrected your problem with re-worked gauges.
Thanks,
JB