I just went through this with the big red I just got. The neutral switch on the motor is connected to the cdi. If the neutral switch is not providing a ground, then the motor will not start. I do not understand how it works that way, but I know it does. Your problem is in your neutral switch. It is on the right side of the motor down low. Once you get the neutral light to come on when it is hot you will be able to start it.
ATCnut
82 185s - sold
83 200x (hopped up)
84 200x (stock)
84 70 (lost this one to the DMV)
85 70 (soon to have lights, a foot brake, and CDI ignition)
84 200es (parts trike) - sold
85 350X (the best)
85 350X (siezed motor, dream of a poweroll kit someday)
85 250sx (frame was trash, got parted out)
86 250sx (the tractor)
86 250sx (a nicer tractor)
86 250sx (an even nicer tractor)
87 200x (parts trike, its really rough, a dog was chewing on it for a while)
87 250es Big Red (the tank!!)