If the new engines have fresh paint, we always removed a spot to paint where the V-6 module are bolted to the blocks. Did you change anything in your main grounding set up from the batteries? How about the ground jump from engine to engine?

Brand new spark plug wires. It did it right off the bat with a new pair of taylor's on there. Even replaced those Friday with a new set of MSD wires that have a lower resistence rating....

Factory location, and they are grounded, paint is not an issue.
No loose wires, or grounds, we checjed every one and polished each one up.....

We did this too.... well not the wiggling it while it was running but we cleaned each prong and spread them out a little so they would make a better connection....

What is strange here, is this seems like the tachs are getting some kind of "interference" at the same time, at the same rpm.... what could cause that?/ We are starting to think about the alternators, maybe disconnecting the purple wire from the alternators, and see if it is cooming from them....

We have had the Mercury manual out for the last two days following the entire wiring diagrams.... We are running out of things to look at.... but SOMETHING is causing this!

I will try wiggling the cannon plug while motors are runnning and have someone watch the gauges this morning.

Please keep throwing ideas our way please! I appreciate it!

We checked that... paint has been removed were the modules are bolted up and nothing else has changed. This was a removal and re-installation by a builder. The ONLY thing different is the blocks are 502 blocks now instead of 454.
Engine to engine ground jump is the same too...

We disconeected the alternators from the system entirely; removed the plug and the wires connecting to it... and the tachs worked..... BUT, the alternators work (are charging at least) because the gauges read 14 volts when connected and 12 when they are not.... looks like this has to be something in the harness to the alternators? the purple wire maybe?

Any thoughts around this? maybe an explanation of the alternators figure into the system? Somehow they send some kind of signal or interupt at about 3800 rpm that make the tachs goof out when connected. Again, thanks in advance for any feedback.

I don't know a solution, but at least you are getting it narrowed down.

Make sure 5 and 7 plug wires are not running right next to each other in the wire looms. They need to be seperated by at least 1 other plug wire.

My next try is to run a ground wire from the dist. cap screw to a knwon good ground. I've been told the distributor grounding can cause this problem.

BT

That is a good one could be some paint on the block at distributor try reseting the timing, turn the dist a bunch of times to get a good seat, I have had to do this before.The purple wire is the ign feed wire out of the alt, I guess if a diode or something is bad it could do this but i have never seen or heard of something like this, But the other part of me says all you are doing is unhooking the ign system main power, But i could be wrong becouse it worked. Did the wiggel test do any thing? What is the temps of the alt, From what I have seen most run 120- 150 degress But I have seen them up to 290-300 in less then too min.

By unhooking the alternators you lower the system volts by 2-3 volts and that alone can make a big difference because of the less amp draw on each circuit.
Hook it back up and check for A/C voltage out the back of the alt if it is more than .5 volt replace it.
Also did you put in new pick up coils in the dist, I have seen this cause a few tach issues before.

I will be down to the lake this weekend delivering the Fountain if you don't have it fixed yet and let me know maybe I can take a quick look.