MOPED ARMY

I'd vote for a corroded switch or wire connector here. Nothing much goes wrong with the flywheel magneto. The light is run by AC from the magneto's lighting coil, and this is produced when a strong magnet on the flywheel passes near that coil.

Corroded or otherwise defective electrical connectors are difficult to diagnose but simple to repair. Assume that every terminal, switch, or any other part of the wiring is defective--that is, it won't conduct electricity--until proven good. Build yourself a 'continuity checker' with a battery, a buzzer or bulb (buzzer is handier) and a pair of wires which, when they touch, will make the buzzer beep. Continuity testers are cheap, too--eBay should have something for a few dollars.

AC generators are inherently simpler and more reliable than DC generators. Just a coil plus a spinning magnet. Incandescent lamps work equally well on DC, AC, or a mixture of both.

But you cannot charge a storage battery with only an AC generator, so that's why bigger bikes use DC generators. To confuse the issue further, an AC generator is often called an 'alternator,' *EXCEPT* for the one that's in your automobile. That one produces DC because it comes with a diode bridge to keep the current flowing unidirectionally.

You may find a test bulb (solder two wires to a 6v lamp) and a battery/buzzer continuity tester easier to use than the multimeter. That's because the test bulb presents a proper load to the circuit under test, while the sensitive voltmeter may show a perfectly fine voltage that you're measuring through a high-resistance defect. A continuity tester that you can hear is just handier to use if it doesn't drive you nuts with the noise.

Note that two pieces of metal, particularly but not necessarily steel, that have been securely riveted together for decades will not necessarily maintain electrical conductivity. That is, every riveted joint, like those in lamp sockets and ignition points and sheet metal fenders must come under suspicion. It's common, for example, to rivet a ground wire to a fender. The joint between wire and fender can corrode and thus open, electrically. In addition, so can every joint, bolted or riveted, between the fender and the ground connection on the lighting coil. Even worse, they can be intermittently open: sometimes they'll work, sometimes not.

The only sure cure is to run a ground wire from the lamp socket back to the magneto. But you can clean and re-rivet questionable joints and that should work, too. This is an excellent application for one of the WD-40-type spray lubricants. My favorite family of these is PB Blaster. Their penetrant stuff is quite aggressive, and I've had good luck with their multi-purpose lubricant.

The Solex had a fuel pump???

Thanks to all, dumb me could not figure out the power feed screw, I thought

someone had played with wires when they added a horn, again many thanks

your input did the trick. Mike.