working on 71 mach 1 with 351c, has a msd distributor and msd 6al box, problem is that the 6al box has output for tach so we tapped into it, and power to the other wire on the tach ( stock tach), and at idle its fine to start but after you reach above 1500 rpm it just dies, then will work again up to 1500rpm if you restart the car, ive also used the tach adapter from msd and that is what im left with, i have evrything else working in the car minus the tach, any input from guys that have same setup that works would be great thanks