I've recently had some trouble with the wiring of my 1968 TR6C resulting in having to buy a new loom combined with much swearing and head scratching. I guess that a similar problem could occur with a T120 or other Triumphs of that era.
The bike had been partially dewired to fit new steering head races and I had also fitted a new battery. Everything electrical was OK without the engine running but when I started it and gave it a brief ride there were clouds of smoke which were due to one of the poxy red earth wires in the loom (14/0.010" - 6A rated) going to the Zener earthing ring burning out. The 35A fuse (as specified) had blown and the incorrect diagnosis I made was that the Zener has
gone short.
To cut a very long and tedious story short I eventually discovered that the problem was due to the seat metalwork shorting to the negative terminal of the battery with the weight of a rider; the blown fuse was a red herring and had only happened because there was lots of molten insulation in the loom.
The combination of the battery positive not being solidly connected to the frame (the bracket to which the rectifier is mounted is ectrically isolated from the frame by the rubber bushes of the tank mounting strap) and that there is no solid electrical connection between the frame and forks means that the small earth wires in the loom can carry excessive current when there is a fault condition.
To avoid the risk of burning out your loom I suggest the addition of at least 30A rated cable to connect:
1. Battery +ve to the rectifier stud AND to the frame.
2. Frame (front tank mounting lug) to the forks (zener bracket bolt).
3. Forks (Zener heatsink mounting bolt) to 4-way female Lucar in headlamp.
A shakeproof washer between the ring terminal and metalwork helps to ensure a reliable connection after a bit of local paint scraping.
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