Check the neutral switch for correct operation. I am guessing that the switch is located somewhere near where the shift linkage enters the crankcase housing. The switch is attached to a wire from the ECM, looks like it is light grey in color and the neutral switch is a simple switch that shorts to ground when in neutral or is normally shorted to ground when in any gear but neutral. Reguardless, the switch body is screwed into the engine and this provides the ground path. Remove the one wire that goes to this switch and connect a ohm meter to the switch and to ground. You should see the switch open and close as you shift from neutral to any gear and back. If not the neutral switch is probably defective.