solex 3800 1973 no voltage to bulbs

Folks, headlight was working but dim, tried horn, then all voltage output

is nil, suspect something under flywheel is at fault, after I remove flywheel

what do I check? what makes the 6 volt dc current? 1st time working on these little bikes, wish it was a plane or car then I would be alrite. thanks Mike.

Bike still does run ok, just no 6 volt output

Re: solex 3800 1973 no voltage to bulbs

Ned Renner /

It could be the magnetto but is more likely something less expensive, like a bad ground or bad switch. Check site below for a simple picture of the electrical diagram of your bike and determine if there is electricity coming from the magnetto. Check moped army’s Wiki and the link to the article I just posted to help with diagnosing the issue. A cheap continuity light and multimeter are great tools.

Re: solex 3800 1973 no voltage to bulbs

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.

Re: solex 3800 1973 no voltage to bulbs

Folks, ok I thought it was dc current like a battery, I guess these small bulbs must work on dc and ac current? I do have ohm meter and voltmeter and I restore vintage airplanes so I am good with elect. but now u say AC so I will go back to some testing

properly this time. thanks for input. Mike.

Re: solex 3800 1973 no voltage to bulbs

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.

Re: solex 3800 1973 no voltage to bulbs

i suck at getting lights to work but how hard can it be? especially on a Solex, i had the same problem attacked it with vengence and still couldn't get it working.

The problem is not with the magneto since your headlight works, powers up the rear light as well. So its with the wiring from front to back. I found a pinch on the rear fender wires near the seat post right where it tucks, and a crack in the wire where it wraps around the front tube, still no luck on my end. It has to be cleanly grounded as well, think theres a short wire in the back light.

Re: solex 3800 1973 no voltage to bulbs

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.

Re: solex 3800 1973 no voltage to bulbs

Make sure you are using the correct bulbs:

headlight: 6 Volt 6 Watt 1 Amp

taillight: 14 Volt AR (7 Watt)

To test if the lighting coil is working:

- the lighting power exits the stator /magneto via this power feed screw:


- You'll need two lengths of wire, stripped on the ends...1 6 Volt bulb...and some electrical tape

- Using the tape, attach the end of one wire to the threaded portion of the bulb, and the other end of the wire to the power feed screw.

Attach one end of the second wire to the tip of the bulb, and the other end of the wire to the crankcase or a fuel pump screw.

- suspend the bulb in mid-air so it doesn't touch metal.

Start the engine. The bulb should illuminate, which means that the coil is working.

There is no ground wire for the taillight. The frame and frame bolts act as the ground wire from the taillight to the engine...there is no all of the bolts and bolt holes must be free of rust and paint to allow the ground to travel through them.

Re: solex 3800 1973 no voltage to bulbs

The Solex had a fuel pump???

Re: solex 3800 1973 no voltage to bulbs

Yes, the Solex has fuel pump mounted to the front of the crankcase, which works off of the crankcase vacuum and pressure.

It pumps fuel from the fuel tank up to the carburetor.


Re: solex 3800 1973 no voltage to bulbs

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.

Re: solex 3800 1973 no voltage to bulbs

Mike, If you'd like a free .pdf Owner's Handbook, a Parts List, and an Engine Adjusting & Disassembly Guide for your 1973 S 3800, just send me an e-mail at:

Also, check out my latest Solex 3800 USA website:

And my BrianSolex Youtube channel:

Playlist of BrianSolex "How to" videos:


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