Engine idles rough when NOT holding brake lever

William Benz /
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My uncle just gave me a 1977 Garelli Gran Sport. It's in pretty good shape for a 42 year old bike, but it's got a few issues.

The one I'm having the most trouble figuring out is that it idles rough - sometimes to the point of dying - unless I'm squeezing the brake lever. As soon as I do that, the engine immediately runs smoothly until I let go again. I only need to pull the lever enough to get the brake light to turn on, so I'm thinking it's an electrical issue.

I've spent a bunch of time reading about the electrical system and looking at the wiring diagrams, but I can't figure out where the fault would be.

I was first thinking a bad ground connection. But my understanding is that the ground one side of the magneto grounds through the brake lever switches, and that pressing those switches breaks the circuit, which then causes the magneto to ground through the brake light. So if it were a bad ground from the levers, I would think it would act like an open circuit and just ground through the brake lights, causing them to stay on. However, I can't figure out where else the fault would be.

Anywhere in particular I should start looking?

Also, any other gotchas I should be looking out for with this bike? It's my first moped, so it's all still a little new to me.

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