Snug it up and then another quarter turn to line up the castellation's in the nut for the split pin. It doesn't need to be THAT tight. If you want an exact torque, 43.5 ft lbs.
close enough
http://www.venold.com/tl1000s/specs/torque_specs.html
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Test the bulb with two wires to the battery.
Check the grounds.
Check the switch.
Then clutching at straws, the continuity of all the wires involved. They often break inside the insulation where the wire flexes from headlight unit/forks to the frame.
take apart. have ultrasonic cleaning done to all parts, add rebuild kits with new jets and then bench synch them, install back on bike and synch them with a good gauge set
