Kenny,
I have the MSD Digital 6A, high vibration blaster coil and tach adapter installed with a Marelli on my 71T (1970 build year), and it runs perfectly. My Marelli, which is original to the car, does not have a condensor or vacuum. The MSD wiring is exactly per the instructions. It's important that the ONLY wires to the blaster coil are the orange and black from the 6A and the engine ground. They are not 12V. The white wire to the distributor gets bussed together with the tach adapter and the tach (more on this below). I don't know if the tach adapter is really required, but it's installed and working. The only tricky part is wiring the tach. Depending on when they built the car in 1970 you may or may not have the cylindrical 'intermediate unit' on the electrical panel. If you do have it then you need to disconnect it and attach the output wire from it (to the tach), to the white wire bus. My car had the intermediate unit. If you don't have the intermediate unit then I assume the black/purple tach output presently attached to the distributor will instead get attached to the white wire bus (it can remain plugged into the distributor with the white wire). Since that is not my setup I can't verify, but it should work. Do not try to use the gray tach output from the 6A, it won't work. Lastly, make sure you have a solid continuous 12V and ground to the MSD. I went straight to the starter and the chassis ground.
If it was running poorly before the install then it makes sense it's not the MSD if it's installed correctly. Points gapped more or less correctly? (not critical with cap discharge system). Timing correct? Distributor mechanical advance working?(35 deg at 6,000 rpm). Spark plug wires good? I would suggest getting new ones, seriously. Carbs adjusted correctly? No vacuum leaks? The racing sounds like a carb problem. I have Zeniths and have found they are extremely sensitive, defined as perfect zero tolerance adjustment, to pressure, float height, jetting and idle adjustments.
Doug
