Vintage Dirt Bike Q & A

Hey guys,

So I've been trying to undo the damage of a previous owner on this bike. I have the whole electrical system back up and running but of course its blowing head light bulbs. Don't worry, I swapped out the sealed beam for a replaceable bulb set up.

I did the popular Dr.Atv Reg/Rec mod in order to solve that issue: http://www.dratv.com/6voregre.html
So I start it up, and again blow the headlight in a spectacular way. So here's the real question: What is in that little black box? If I failed to ground it correctly, could I have fried it?

A multi-tester on the headlight socket reads 35V with the motor revving, and over 12v at idle. And that is with the Reg/Rec plugged in! If I read the wire diagram right, the yellow wire Y's into the headlight hot wire. The green wire goes to frame ground. The other green wire goes to the lighting coil output. The red goes to battery +.

Anyway, thanks in advance.

Woody

and one more thing, how do I know if the bulb is going to blow, until it does? And if the bulb blows, and I am running the original resistor, how do I keep that from going up in smoke as well?

Woody

Woody,


It's probably a rectifier diode or diode bridge, along with a high-wattage Zener diode and resistor. If it's properly designed, I don't think a disconnected/misconnected ground would fry anything.


Here's how I read it:

1) Connect the red wire to the rectifier output wire (rectifier is removed).
2) Connect one green wire to the rectifier input.
3) Connect the yellow wire to the lighting coil output.
4) Connect the other green wire to frame ground.

I don't know what's up with the two green wires. A continuity check would tell you if they're connected together.


Remove the bulb and check the voltage. It's OK to see a bit more voltage than the bulb rating as it will probably drop a little under load, but 35V on a 6V lamp will blow it in a spectacular fashion.


A voltage check will work here too, as will a finger heat check on the resistor (warm to very warm is OK, burning hot is not). Power resistors are pretty rugged if they're rated properly for the job they have to do.

Have you gone over the entire wiring harness carefully? If the MT125 is anything like the MT250 - as I assume it is - there's a ton of wires, connectors, switches, bulbs, etc., any of which could mess you up if connected wrong.

Ray

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

Hey Ray what would make the resistor run too hot I burnt the crap out of myself on mine left a real nice blister on my knuckle so it was a bit more than warm. I am gonna check the grounds and everything as I am unsure what would cause this. I also was wondering if its normal to blow the bulb if the bike is revved to high mine was working fine tell I hit 9000 rpm and it went out. I dont normally rev it that high it kinda got away from me a little bit lol when it hits the power band it really flys.

During normal operation, the resistor and light bulbs share the output from the lighting coil. If the bulbs are the proper wattage, they soak up most of the power and the resistor gets the rest, acting kind of like a half-assed regulator. If the bulbs are incorrect or burnt out, the resistor will be forced to dissipate more or all of the coil output, getting hotter in the process.


In a brand-new bike, I would say no, it's not normal to blow bulbs on over-rev. In an older bike with probably all sorts of age-related changes to the stator, resistor, rectifier, bulbs, etc., it seem quite likely to have overly-bright lights that burn out quickly and/or at high rpm, too-dim lights that work like crap and last forever, or anywhere in between.

One other thing... if a bulb is being operated directly from stator AC, adding a rectifier diode in line with either bulb wire will convert that bulb to DC operation, reducing power to the bulb by about 30%. This is what's going on inside those disk thingies that drop into light sockets to extend bulb life. Using 12V bulbs may also work well.

Ray

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