Vintage Dirt Bike Q & A

I just found this site after purchasing a '74 MT250. I got the bike running pretty well after cleaning out the carb and replacing the intake gasket. The problem: I have no electrical power going to any of the components. No headlights, no horn, no neutral indicator light, and no speedo/tach illumination. The handlebar control switches were broken, but I did manage to get the lighting to the "ON" position. The rear tailight has been removed, only the plastic connectors remain. The turn signal lamps have been removed as well (the wires were cut!.) Oh ya, the bike did not come with a battery and I don't even see how one would be connected as there are no terminals/wires present.
Shouldn't I get power to the headlights/instrumentation after the bike is running, even without a battery? Do I have to have a battery installed for the whole system to gain power. Does anyone have an idea why I have no power when the bike is running normally? I know this can be a can of worms but I was just looking for some ideas...

These electrical systems are complex in terms of the amount of wire involved, connections, fuses, and grounding. If you want all the lights to work as from the factory, you'll need to trace your wiring harness for continuity and grounding, as well as have a functioning battery installed. You may have to replace some or all of the wiring harness, check/replace fuses, install new controll switches (all available on ebay). Get a shop manual for the proper wiring diagram.
If you just want a headlight and nothing else; you might try straight wiring it to the lighting coil (When you start the bike the headlight is on). It'll burn fairly bright as long as the bike is not idling. Not enough juice to run everything using this method.
Hope this helps.

Thanks for the reply. I know wiring is a crazy task to get involved with, but I really only need the headlight to run. I am using the bike as a scouting vehicle and I need the light for the early morning and dusk hours. The lighting coil on the stator should already be directly wired to the headlight, no? I already checked both the master light switch and repaired the hi/lo beam switch.......so hopefully after the fuel tank is resealed and she's running I will know if I have juice to the front. Thanks again!

PROBLEM SOLVED
The HI/LO Beam switch had broken internally.....previous owner probably dropped the bike and smashed the housing....all I did was reconnected the metal leads inside the switch housing...and BAM....got a light. I can even generate enough juice by simply kicking the kick start rapidly. Once the bike is running again the light will work perfectly. Thanks to all of you who helped me with this one!!!

Glad to see that you got it working.

Just one pointer....see my post under your other post 'Wiring dig."

Great! Things worked out incredibly well, especially considering what you started with.

An excellent point was made about the lack of regulation in these simple lighting circuits, which gave me an idea... perhaps you might want to try connecting the two headlight bulb filaments in parallel, which would give you a much brighter headlight than stock, and would also serve to load down the lighting coil better and reduce the voltage swing from low to high rpm. Since the switch is already broken, it wouid be a simple matter to connect the blue, white, and blue/white wires together to make this happen. Also, the MT250 has a 3.6 ohm, 10 watt resistor which adds a bit of "regulation" (really just brute-force circuit loading) to the headlight circuit, probably to guard against bulb burnout in the event of ending up with just the headlight, as you have now. Still, it might not be a bad idea to add the tail light back into the mix if possible - you might be glad you did if you ever find yourself leading someone out of the woods on a dark night.

If you get the chance sometime, it would be a very good idea to take an AC voltage reading across the headlight while you're revving the motor. You might be surprised at what that poor 6V bulb has to contend with.

Ray

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'74 CR125M (175cc), '75 MR175, '82 RM250Z, '08 YZ250F, '14 Zero FX electric, '14 Zero MX electric, '18 Alta MXR electric

So...
If I were to leave things just as they are, do I run the risk of popping my headlight bulb if I get really high into the RPM's? I really am a poor mechanic when it comes to anything with a wire...so I was kinda hoping to get away with just the headlight (which is all I really want).....Trust me, when I get around wires there's no telling what mess I can get myself into.

I just thought of something...
So the lighting coil in the stator should be running all of the front lighting systems...right?
So far (using my rapid kick method) I only see the headlight coming on very dimly. Shouldn't I start to see some dim light coming from the tach, speedo, and neutral lights? Am I not creating enough current to light all of the systems yet? I guess I will have to post again once the bike is back and running (that is another story altogether..rusted tank, exhaust issues

Let me know what you guys think. If this observations persists even after the bike is running could it mean that these other front bulbs have popped and I only have the headlight? I thought it was perfectly okay to run the system w/o the battery and leaving the rear system disassembled.....but then again I know sqaut about electrical...so educate me!

The kickstart thing can tell you if the big bulbs are working to some extent, but in my opinion it's not a very useful troubleshooting technique. If you're not comfortable using a meter, I would suggest manually checking all bulbs for broken filaments for now, and then waiting until the motor is running for any further investigation.

Just FYI, your headlight is still available if it ever needs replacement. Apparently it's a large sealed beam unit, though, so it's not cheap (about $54).

Ray

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'74 CR125M (175cc), '75 MR175, '82 RM250Z, '08 YZ250F, '14 Zero FX electric, '14 Zero MX electric, '18 Alta MXR electric