The tachs worked before you changed the engines so it can be one of two things. As Zastrow says it may be a ground. Take a seperate ground wire from the negative side of the tach directly to the battery ground. It also could be getting an interference noise from the blower. Livorsi sells a filter to filter out noise to deal with this problem.

As for the GPS the antenna battery could be bad. If you send them to Livorsi or Elite we can check for any of these problems.

My engines were pulled and the same thing happened. I have not found the problem yet but mainly due to being lazy and the water calling

I had the same problems with Livorsi tachs all last year. Swapped tachs, changed tachs, sent tachs in for testing, redid all grounds and ran new grounds. Still crazy tachs, one side worse than the other. The only noticeable improvement I got at one point was changing spark plug wires. But is still was not fixed. I added blowers last winter, so maybe I need these filters. How do they install?

I had a similar issue last season. My stdb tach would flutter and the engine was skipping at higher rpm's. After a bit of troubleshooting, we found the problem to be a loose ignition module ground.

The tach suppresor goes in line on the signal wire. It is a simple install. Also bad spark plug wires could add to the signal noise on tach and will give it iratic readings.

Can you give me a part number and contact for purchase?

This is everything we tried:

used an MSD tach generator, simulating engine running, tested good, tachs worked fine to the rev limiter.

Other grounds at gauges were checked; all were good, showed 12.9 volts at each gauge positive to negative.

Also did a sweep test on each gauge by grounding sensors on both motors to a known good ground, function tested normal.
Sytem voltage is 14.2 volts on both engines idling.

Hooked a known good ground wire to every ground possible on both engines, did a test run after each connection.....

Tachs work great right up to about 3800 RPM, and they both STILL start fluttering and go back down toward zero.
I am really at a loss here....
Anything I am missing here, this is really baffling me here, more suggestions would be much appreciated....

Thanks. Mark

This may seem like an odd way to check your tachs, but see if one of your friends wouldn't mind putting YOUR tach in his boat. If it works (which, from your MSD generator, would suggest that it will work just fine), you have ruled out the tach. One of my boats had a similar problem where the tach and water temperature gauge (but none of the other gauges) were fluttering/spiking and we checked everything ... except a couple months later the alternator literally fell apart ... the long screws that held the cases together had vibrated out. Made a mess of the alternator, but, when it was rebuilt, the gauges quit their eratic traits.

Did you use new spark plug wires with new engines or reuse the old ones????

What are the ignition modules bolted to????

X 2, start the engine up then go back and wiggle the main power wire harness and watch all the gauges and see if they wig out at all or some times the engine will die.