Yellow wire is the head light and black wire is a ground
Now,as to the idle. the back wheel should spin "slowly" ,to adjust the how fast the wheel spins you adjust 2 things
the carb needle and the idle screw
Try this,
Carb needle 1st or 2nd notch from top(less fuel on idle the higher up the needle)
and idle screw 2-1/2 turns out from tight(more air at tight, less air, out/loose) GOOD LUCK