Vintage Dirt Bike Q & A

Hey everyone, brand new to the site. Great to know people still care about these great bikes. Hoping to get some help. I have a 74 mt250. I changed everything to 12V when my regulator/rectifier went & blew out a bunch of bulbs & blinker relay. I even got the correct resister for the new headlight. It worked great in the daytime but the bike can't produce enough power at night to power the headlight, brakes & blinker, for example. Anyone know of any honda 12V coils that I can modify to fit or re-wrap them? I'm open to ideas. Was looking at the mid 80's coils to fit in but can't be sure without the flywheel size. Thanks

Pvl has a cdi they can make work on the mt250 the same one used for the cr250m and it can be supplied with a 12 volt circut for lighting. You can also contact ricky stator and see if he would rewind your lighting coils to produce the proper current. I have been told he will do it for the honda xl's from the 70's so its worth a shot.

Thanks for the reply. Seems like it'd be easier to have the coils re-wrapped. If it was done, it'd be ready to go with the 12v system? or would I have to buy anything else to complete the conversion?

what have you done exactly at your conversion and why the resistor for the headlight?

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80 CR250Ra
80 CR250Ra
78 CR250R
80 CR80
82 Bultaco Sherpa 340
78 Bultaco Pursang 250 MK12
76 Bultaco Frontera 370 MK10
76 Bultaco Alpina 250
73 Bultaco Lobito 125 MK6
76 Suzuki RM125A
90 Kawasaki KX250

new battery, all new bulbs, regulator/rectifier, new non-sealed headlight, blinker relay, fuses & high-beam resister. I think that's it, well re-wrapping the coils too ha. The resister is on the bike to help with the high power demand from the headlight. It's on the factory wiring schismatics too. Makes it a great "old" bike to ride, while also easy & cheap to fix with parts readily available in town for you.

ok, i checked the wiring diagram.
the headlight has it´s own coil in the generator.
this produces only the exact power what the 6V bulp needs. this will be ~40W (35W for the highbeam and 5W for the h/b indicator lamb)
the resistor prevents the 25W lowbeam for killing it. in this circuit theres no recifier or regulator, so it´s AC.
no chance to get a 12V bulb working right.

the rest electrical (exept of the ignition) gets its power from another coil, the recifier makes DC and the battery holds the tension to 6V without a regulator.
now you changed bulbs and battery to 12V, i don´t think the generator gives you enough power to feed all the bulbs and hold the battery full charged. i guess, if you have a longer ride at night the battery gets empty.

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80 CR250Ra
80 CR250Ra
78 CR250R
80 CR80
82 Bultaco Sherpa 340
78 Bultaco Pursang 250 MK12
76 Bultaco Frontera 370 MK10
76 Bultaco Alpina 250
73 Bultaco Lobito 125 MK6
76 Suzuki RM125A
90 Kawasaki KX250

Yup haha that's the problem. You can stall it at night if you have all the lights going at low idle . I thought even with the lowest wattage bulbs, it'd work. Or the battery would take most of the brunt & charge back up while cruising... but my multi-meter told me a different story

Just for grins, you might try connecting the lighting and charging coils in series, and see what you get. From the diagram, it looks like you could unplug the white/yellow (headlight) and pink (rectifier AC in #2) stator wires and connect them together, then connect the other side of the white/yellow to the black-wire bus (DC positive) and ground the pink rectifier wire to the chassis (it's possible this wire would have to be run to the grounded end of the headlight coil, which would then be lifted from chassis ground). If the resulting voltage was very low, the yellow (rectifier AC in #1) and pink wires from the stator could be reversed. The headlight resistor would probably have to be disconnected.

I haven't exactly done an in-depth circuit analysis on this mod, so there are most likely things I've missed that also need to be done, but the basic idea - to run everything from 12 VDC - seems OK.

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