A mod motor will draw too much power for a 12 volt supply. From the GFX manual:
SUPPLY VOLTS- The GFX will operate from any DC supply within the voltage range of 12 to 16 volts and 20 amp or more capacity. It will operate with less amperage, but the charge current and motor run will be limited to the maximum capacity of the supply. A 15 volt supply is required to charge 8 cells.
Get yourself a 30 Amp Rivergate Power Supply, and you won't ever have this problem.