Engine idles rough when NOT holding brake lever

William Benz /

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.

Re: Engine idles rough when NOT holding brake lever

♣Slew Foot♣ /

Generally it's one of two things either a one of your headlights or light is out to inside of your housing of your tail light there is a ceramic resistor it's broke or it could be a resistor inside the headlight bucket on the wiring block

Re: Engine idles rough when NOT holding brake lever

i'd blame that ceramic resistor, or wrong wattage bulbs. possibly also dirty points. Basically the issue is that the coil for the ignition grounds through the brakes. When the levers are released, it's grounded through the brake switches, and the ignition still sparks. When you pull the levers to brake, it breaks the ground and the power instead goes through the brakelight bulb, causing it to light up, and it is grounding through the bulb (and/or rear resistor) and still sparking.

The most common problem this creates is if there's an issue in the bulb or wiring, then it can't ground through the bulb, and pulling the brakes makes spark cut out. It sounds like you've got the revers issue, that it's grounding better through the bulb. This could indicate poor grounds or connections up at the brake switches, or instead it could indicate that it's running better with resistance (the bulb) in the line. This could be the case with dirty points for example, the increased resistance is encouraging power to bridge the resistance of whatever tiny corrosion layer is on the points? Note: i'm being completely speculative here, but it makes sense to clean your points and your connections and grounds.

Re: Engine idles rough when NOT holding brake lever

The other "gotcha" or important piece of infor with garellis is never use ATF for transmission fluid, only ever use SAE ND 30 oil (or other "ND" oils. ND is for non detergent, detergents in some motor oils and especially in ATF cause the rubber clutches to swell and create issues.

Re: Engine idles rough when NOT holding brake lever

♣Slew Foot♣ /

You can also check to see if your brake sensors work

Re: Engine idles rough when NOT holding brake lever

William Benz /

Thanks for the help! This gives me some good starting points. I should have some time this weekend to dig into it.

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