My port tach is ok to 4000 rpms but over that the tach goes nuts like it has a short in it. I changed tachs and still has the same problem. The engines are 454 425 HPs with factory ignitions, any thoughts on this would be a great help!

I had a similar problem. I solved it by buying electronic filters from Livorsi, and it solved the problem. Never had the problem since. If you give them a call they will know what the part number is.

I had the exact same problem at 4K. Posted it on Do It Yourself-Boating on a Budget and got some suggestions to ground the distributor and another one to make sure the coil mount bracket has good ground and also coil + terminal wire is isolated from the prime ingnition wire. I've done all of those things but won't be able to test the results til this weekend. I'll come back and let you know if any of it helped or not. If not...I'm calling Livorsi to get one of those filters GS42fun got to fix his problem. Good luck from Lake Texoma where the Blue/Green algea is trying to ruin my Labor Day.

Thanks I'll give them a call.

I had the same problem with a brand new set of livorsi tach's , tried everything ,filters,differnt ground wires, resistors, and a shielded cable nothing worked so i tried a set of non electronic ones (old style) and they work great

Thanks for you input. We trouble shot the tach last night,tried the stb tach on the port still the same,ran a wire from the V6 module to the tach still the same,changed the V6 module still the same, double grounded the tach still the same only thing left would be the distributor. Also changed coils.

I have the same problem!..
My boat originally had the Thunder IV ignition, in tandem with an MSD. The MSD failed so we bypassed it to use only the Mercruiser Thunder IV. Ever since this bypass, my tach goes crazy over 4K rpm, just like yours. Let us know if you find the problem.

Thanks!

Julian

Ill let you know, I will post the answer as soon as I find out.

Any luck yet? I tried every thing suggested in my post for the same exact problem and had no luck.

i had exact same problem, with both mine, at the same time.. thought it could never be they both went bad at same time. finally bought 2 new ones and they work great