just installed a msd box and coil.here is my problem at idle puffs of black smoke out the tail pipe.as you bring up the rpm's it starts to smooth out but then some times it starts to back fire out the tail pipe.plugs are gap at .55 like they said to do.when i disconnect the box the motor runs perfect. any suggestions.the car is a 1987 ford mustang with the 2.3 4cyl motor.