Back fireing duraspark?



:( I am using a duraspark out of a 78 Farmont on my I-6 200. It runs great untill I put the heater fan on high. Then it will back fire when I try to start the eng? Turn the fan off and it starts fine? I have check the wairing and it is correct. (It gets power from the key when its turned on.) It looks like its retarting to much, and dosent have enought amps with the fan on high???? How can I get a better amp flow when I turn the key on???
Help!! and thanks Jim.