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1972 GTR125 electrical issues

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R.C. Rick Carlin
Austin, TX, USA   USA
Hi,
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.
R.C.

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nomadinsiam Orpheus Stephens
Bangkok, Bangkok, Thailand   THA
In reply to # 1517 by R.C. I find this website very confusing to maneuver because it has too many tabs to explore.

You got that right. I just joined five minutes ago and I'm already posting things to get myself banned. Once I'm signed in, when I click to a thread, it asks me to sign in again to view a posters sequence of photos. That's a bit redundant I think.

Sidecardriver Richard Placak
Atascadero, CA, USA   USA
I have had some similar problems. I know that if the pilot light in the headlight shell doesn't have a lamp in it the tail light will burn out.
My tail light also goes out when I actuate the brake light switch. I also would appreciate help with this problem.

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hjarvis Avatar
hjarvis Hugh Jarvis
Buffalo, NY, USA   USA
1974 Vespa 125 GTR "Isabella"
It may not be a short....

Some basics.

Always have all the bulbs in place and make sure they are correct wattage (and voltage!). There's a balancing act where the total wattage used must be close to what your engine is putting out. You can vary a bit from the recommended values (e.g. 10w can be 12w).

Look into mounting a voltage regulator (AC 6v) in your circuit if you don't have one. The voltage on scoots without a battery can really fluctuate a lot, and if it gets too high, you start blowing bulbs, which means the rest of the circuit gets even more overloaded. (cascade failure)

Get a bunch of spare bulbs, because you will need them.

Look into switching to a better headlight (halogen) that is brighter and more durable. When the headlight goes, the other bulbs go FAST.

Get the manual. I have a '74 GTR running 12V electronic CDI and a Stella 166 upgraded engine, but I use the closest manual to mine, which is for several models (Rally/Sport...). But it helps.

Look into switching to 12 volts and electronic ignition. REALLY WORTH IT.

ps. I gave up on trying to get readings with a meter. It's hard to read ACV as if it is DCV.

Good luck!

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