Motorcycle Repair: 1975 shovel dying, rear fender, rear harness

Question
Hi Roy, I have a 75 shovel that i bought recently and love it. The only problem i have is when i bought it the rear tailight was not working ( a shit maltese led type),I saw a loose earth wire and hooked it up and this seemed to work for a while.I was out cruising on sunday arve and the bike started to splutter and backfire then died. I switched the lights off and it started again as soon as i turned the lights on it died again. I then rode home with no lights and have been racking my brains ever since!!!!Any ideas bud

Answer
Justin it sounds like you have a chafed hot wire somewhere in your lighting system.  When you turn your lights on they are sucking so much juice that you're losing power to your ignition or getting a weak spark.  It could be just that your battery is low.  Put a voltmeter on your battery terminals.  Fully charged it should read about 12.8 to 13.3 volts.  Turn on the ignition switch and see if the voltage jumps way down.  It should not.  Turn on the lights.  The voltage should still not jump far down.  Now start the bike.  The voltage reading with the engine running should be about 14 volts, or at least more than battery voltage with the engine NOT running.  Turn on the lights. Voltage should not go far down.  If it does, you're not charging.  All these tests should be done after fully charging the battery up to at least 12.8 volts.  Might just be a bad battery, bad battery connections, not charging, or a chafed wire in the taillight harness.  Most common is under the rear fender where the harness has broken free and chafed against the tire.  If you can unplug the taillight harness at the front of the rear fender and then run the bike and turn on the lights it will tell you if the problem IS in the rear harness or taillight or somewhere else like under the gas tank or in the headlight harness.  Good luck.