An NU50 needs a 12VDC power source to provide power for the CDI black box (capacitive discharge ignition)
CDI Black Box wiring: SW = Blue/Yell , EXT = Black/Red, PC = Black/Yell, E = Green, Ign = Black/White
Black/White - goes to the ignition side of on/off key switch and to the stop switch (when off, grounds out the ignition)
Green - ground
Blue/Yell - ignition trigger/pick-up (in alt)
Black/Red - 12VDC power source
Black/Yell - coil primary side
Also, you need a 12VDC battery for the lighting/aux power systems. The white lead from the Alt coil goes to the SCR (silicon control rectifier) to convert AC Alt power output to DC Battery input (charging).
The battery alone will not support the horn, turn signals, lights at idle/low rpm w/o discharging the batt during nighttime operation. The lighting coil tap in the same Alt coil (yellow lead) is to provide auxiliary power for these functions.
See also Hobbit CDI Wiring