Tach help

I got a 66 dart with 360 in it. Got electric ignition with stock ignition box and mad electrical amp meter removal. Now the problem I have is when hook up a aftermarket tach it won't rev past 4000, remove the tach and its good. I first use a tach with a shift light. Then got a plain Jane tach and it still does it. does any one know what cause or fix Is it back feeding in to the ignition box. Never ran into this. Thanks