I just installed stainless marine exhaust system to a set of 502MPI's with captains call. I sent my computers to Arizona speed to be re-mapped. I also added 1.8 roller rockers as well. Other then these 3 upgrades the motors are stock. Now my tachs work properly up to 3000 rpm and then they start bouncing around the dials. The engines are running fine and all other guages are working normal. My mechanic is showing it as some sort of RF(radio frequency) problem. We've checked all the grounds, tried different spark plug wires, and added noise diodes to the lead tach wires nothing worked. Anybody have any ideas ??? help

stock ignition or are they msd's.

and how bout the distributors

I had this problem on my stock 500 . Was told by Lavorsi to run a ground wire from the dist to the engine block. Was told the intake piant causes some problems with grounding. I did this an have had no problem for three years. Unitll this spring after I had some work done. But I see my guy didn't put the jumper wire back on. So I will do this.

Just FYI, you problem may be diff

they are stock ignitions/distributors. We did put new wires,plugs, cap and rotor (basic tune up)

they are stock ignition and diistributors. We did change the wires,plugs,cap and rotor (basic tune up).........

The only new electrical component is the Captains Call, and they use a large capacitor and a constant duty solinoid, so the potenial for EMI/RFI is there. Maybe try disconnecting them and retest.

does this occur when they are NOT engaged.

The boat already had thru hull exhaust with captains call from the factory and the tachs worked fine. All I did was replace the cast iron exhaust with the stainless marine exhaust .

not engaged, meaning the captains call ? If so yes

yes.