I had the exact same problem and never did get it corrected. I went as far as running a new signal wire by itself to the tach and running the tach off an independent battery, changed plug wires, running a separate wire from the distributor to the block to ground the distributor. This was several years ago and after trying multiple tachs from Gaffrig I just gave up. Whats funny is I have a digital tach mode on my gps speedo and it works fine. If you get them to work please post your solution.

I brought my boat to my marine mechanic and he checked all grounds and connections still the same problem. He also installed tach filters from livorsi and that didnt work either. He hooked up an old analog tach and that worked fine. We are both confused. I picked up the boat so I can use it for the weekend and then I will bring it back. Any other ideas?

Any fix yet on this? I have one motor doing the same thing checked all the grounds all good changed modules same problem. Maybe it's the wires?

Mark did you try switching the tach or wire to the tach I should say? I had a tach do this years ago. I had to turn the selector on the back of the tach, for 4,6,8 cyl, back and forth several times. Whatever that did, made it stop acting up.

Old threads with good info.

http://www.offshoreonly.com/forums/g...s-500-efi.html

http://www.offshoreonly.com/forums/g...ng-around.html

See post #9 of this thread.

If you guys would just stop revving the engine the tach will stop jumping....

NEXT!

I had the same problem..Nordskog gauges.Merc thunderbolt IV ignition. I tried everything, ran a new wire, changed the tach, checked all grounds. Problem solved when I switched to MSD ignition complete with new distributor

The Cure
 

I had the fluctuating tech problem on my Formula 353 w/ twin HP500 EFI's. Went through all the troubleshooting stuff discussed here: swapped tachs, plug wires, checked grounds till sunrise, etc.. No joy until...

Turned out the problem was the ground connection between the distributor base and the block. The issue was caused by the paint job on the block which insulates it from the distributor base. The easy cure, suggested by to me by someone at Mercury Racing, is a jumper wire between the distributor cap hold-down screw and and engine-block ground. Worked like a charm. Its dead simple hook up a temporary jumper to validate that it solves the problem for you.

This may also cure other guages jumping around as revs increase through the 2000-3500 rpm range.

Note that the engine will operate flawlessly without the distributor-base grounded to the block. This grounding apparently serves to ground out electrical noise from the ignition appearing on the tach signal wire.

We ended up putting a pair of older gaffrig tachs in when a friend redid his dash and they work fine! I plan on just leaving it alone until I redo my dash.