Vintage Vespa Forum

1972 GTR125 electrical issues

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R.C. Rick Carlin
Austin, TX, USA   USA
I'm new to this website and haven't been successful in finding what I'm looking for. I find this website very confusing to maneuver because it has too many tabs to explore.
I have a 1972 Vespa Gran Turismo GTR125 that has some electrical issues.
I got it running after cleaning out all the gummed up fuel from the tank and carburetor, but it quickly burned out the rear brake light 6 Volt 10 watt bulb on the first test run.
When I apply the rear brake pedal, the taillight bulb goes out, so I believe it is shorting somewhere. There are a blue and yellow wire running to the tail light. Blue is for the brake light and yellow for the tail light. There is no brake light switch on the front brake handle bar lever.
I don't want to replace the bulb without solving the problem, and when connecting my Voltmeter to the rear brake light contacts, it flutters all over the place, so I can't even get a good reading.
This scooter does not have a battery so I'm not sure it even has a rectifier or if the lights run off of A/C Voltage and what the acceptable voltage range is between idle and high RPM. The wiring diagram I found doesn't show a rectifier or regulator.
Is there a 12 Volt low watt replacement bulb that can be used in place of the 6 Volt bulb?
I also am unsure about the headlight wiring. Which element is supposed to be for the low beam?
The bulb has a shorter element that is unshielded, and a longer element that is shielded.
The bulb can only be inserted one direction and the longer shielded element faces up.
The high beam indicator light works, but the headlight is much brighter on the low beam setting, leading me to think it is wired wrong.

Thanks for any help.

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