Brad - I presume your B has been converted to 12 volts?? If so, you're overpowering the old (6 volt) coil. Solution is to install a ballast resistor in series with the coil primary to drop the voltage at the coil to 6 volts. Insert the resistor in the wire between the ignition switch and the coil "+".
Alternatively you can replace your 6 volt coil with a 12 volt one, though this will cost more than the ballast resistor.
If you go the ballast resistor route, get a generic (Chrysler style) resistor - NAPA #ICR13 or similar. Be sure to mount the resistor to something metal on the tractor (NOT the gas tank!) as it gets quite warm when operating.