So I did the mefi3 to mefi4 conversion harness on my 502 (kept the mefi3 engine harness) and the tachometer would go to 4,000 rpm and then drop to zero until the engine dropped to about 3500 and then it would start working again. I bought a new tack and hooked it with jumpers right to the negative side of the coil and the same thing. I looked up the pin out for the mefi4 and on the J1 plug, pin 14 ,ecm tach driver circuit. The jumper harness did not have a pin there so I pulled the pin from the fuel psi reg as the wire for that goes to a 10 pin plug that is not used for my application, and hooked the spare gray tack wire to it (pin 14)and removed the gray wire from the connector that goes to the coil negative from the engine side of the main harness plug.

I could not find any info about the tach problem. The guy I got the jumper and mefi4 from said it did the same thing on his boat and could never get it working. Hopefully this will help people out in the future.

Tim