I had a bad carb once. On a new motor. I couldn't get the car to run right. It was very rich and the car wouldn't idle. The plugs were black. I pulled the intake and resealed it. Still nothing. Then I got a known good carb. Bolted it on and the car ran perfect. I would get a known good carb and try it. If that carb runs the same it is not the carb. Then I would get another MSD box. If that doesn't correct the problem I would look elsewhere but most likely one of the two should do it.