1972 CB125 Project

[Guest]

Sorry for the late reply, I don't get any sort of notifications so it's best to pm me if anyone has a question.

Yup that rectifier is fine and has a nice mounting hole in the middle to make the metal it's attached to a heat sink.

Rectifier: The post that is twisted or different than the other 3 is the positive and directly across from it diagonally is the negative, and the other two don't matter for the orientation.

The pink wire on Honda's is typically the charging coil circuitry and the yellow or white the lighting coil circuit off of the stator. The difference is charging uses both coil windings and lighting just one.

The charging usually kicks in somewhere around 3k rpm, so beware of battery drain on these older bikes just starting them up for tests and checks, because unless you hold the throttle at 3ish K you're not charging anything during those tests. It's also why many people think you have to rewind a stator for a 6v to 12v conversion.

Typical stator output even with a stator from a 6v bike generates over 10,000 volts more than enough to charge and burn a standard H4, if there is flicker when stopped or idling it's likely because the lighting circuit and charging circuits are wired improperly if the bike is already 12v. But if there is a lower output rate on the headlight, then either running the lights ground all the way and directly to the battery negative or adding a lighting coil will stop it. A lighting coil can be made or found on the cheap as they were in use on 100's of old mopeds.

[Guest]

Thanks for that!! Appreciate it.. Excuse my ignorance... I have a red, yellow, Pink and green going into the Rectifier.. Green ground? Red live? And the other two are interchangeable? So once I do this and hook up the 12v batt. (and bulb) I should be good? Am I missing anything else? Is there any test I should do before I power so I don't blow the coil or something?

as long as you don't leave the switch in the "on" position when the bike is off for over 30ish seconds your coil will be fine unless it was already on it's way out to begin with look for shit that like looks like wax drippings out of the coil none? Then you're good on the coil just mind the key switch position... simply because on these older bikes the "on" pre-energized the coil.

Any 12v coil will work on a single paying attention to ohm's if you care about being fcc interference compliant, otherwise fuck the ohm rating. When I need a coil for an older 2 cylinder I typically just get a used one from a 4 cylinder bike made in the 80's or 90's... basically whatever is fucking cheapest and looks to be in good shape.

[loser69]

I forgot I made this thread haha I haven't done anything with the little Honda since I made this thread, still plan on it though been too busy with a Triumph and now an old Harley. I would like to make it a cheap little legal off road machine. I'm glad the thread has been useful!

[dereck]

Awesome guys!!! I am going to dig in now and report back!! I think I just enough knowledge now to be dangerous!!! :P

CraiGORE! that is a sweet CB!! That is my next build.. A twin!!!

[dereck]

WOW!!! thanks guys I think I did it!!





only question... How do I know if my battery is charging? Any way to test? I know the battery is good, just don't know if my bright lights and loud horn is just running directly from the batter and if it will all end once my battery is drained. Any way to ck the charging? Don't know if it is my imagination, but it actually seems to be running better!!! Very peppy!!

[dereck]

sight and sound!!!

[bbcommo]

looks like progress, my buddy has a little cb125 down the street, im thinking about doing some fencework for him to trade it out. If that happens i guess ill have a nice starting point.

[ruckus2115]

nice little fun build. Looks good. have any better side profile pics of it? what all did you do with the engine rebuild?

[Bigvin]

Help! CB125 Charging System.

Hi Jetblack. I am very interested in your post, as I have put CB 125 engine in a Honda CR80 rolling chassis for my daughter. The bike is great in many ways, but has been a bit unreliable due to the electrics. I was about to tear my hair out before I found your post. When I initially wired it up and got it going, I was measuring 20-30 volts across the battery. I have fried two dry state batteries and boiled dry one lead acid battery. I replaced the rectifier with a rectifier/regulator, which was fine for while, although it ran very hot. So hot in fact that you couldn't touch it, and the plastic protector melded and dripped off. Now it is not charging, so the battery goes flat. So I replaced the rectifier/regulator with what turns out to be just a rectifier off ebay. Now reading 20-30 volts across the battery again. I do wish people would described their stuff accurately!! So I am very keen to convert the system as you suggest. The wiring system I have used is as shown below. Hope you can see it when I hit post. My questions are: Would this rectifier do, see ebay item 121680145216? What is the spec of the capacitor I would need in terms of volts and microF. How is the voltage regulated? Or doesn't it matter? even though I will be getting 20-30 volts in the system?

Conversion is easy peasy, I convert all my 6v bikes to 12v when I get them. On old 6v systems the battery acted as a regulator as in excess was sent back into it, well the idea didn't really work so well because anytime you were under maybe 4 to 5k rpm all lights were being powered just off of the battery storage, when the bike got at that RPM 5k ish it typically generated around 10k volts to quickly charge the battery back up...

The bad news is if you stick with 6v you'll always have a weak shitty electrical system. The good news is, since the stator output so much voltage it'll maintain a 12v great, of course the old rectifier has to go, a radio shack dual bridge rectifier is cheap and does the job great even has a hole in the center to use for mounting and costs under 5 bones. Don't need a regulator since the excess voltage is used to charge the system.

Hooking the new RS rectifier up: The bent post is the positive diagonally across from it is ground/negative. You'll have 4 wires off the old rectifier look at a diagram to find the positive rectifier wire hook it up, either jump a wire to ground or hook to negative from diagram if one is present diagonally across to the other post, the other two wires do not matter which post as both are AC outs.

Here's a picture to aid what I'm talking about:


Next, change all of your bulbs out to 12v, and use smallest drawing headlight bulb you can find no more than 35W is best, and if you can hook LED's to the other lights the brighter the head beam will be, less chances of dimming or flicker at idle and low speed due to no charging taking place... from low rpm, the secondary stator winding does the charging, the primary winding keeps it running.

If you're using blinkers you need a 12v flasher, and you need to bump the main fuse up to a 15A with the conversion.

Power source: You can use a battery or a large capacitor make sure the capacitor you use is unidirectional meaning the voltage only exits out of one post not both.

And well that's all there is to it.

Benefits: Faster starts, charging RPM drops to about 2K range instead of the 5k range, horn much louder, and your not gonna play nanny to a battery, and you can hook in a 12v accessory cigarette plug if you want to.

Some see this as a negative: Starter spins faster if you have one. I don't see it as a negative, faster spinning flywheel equals faster start equals less wear in the long run or at least balances it out instead of dragging it with 6v.

My two current bikes with the conversion:

My 150cc CA95 can sit over 8 months and still be holding 12v or better... too bad gas can't do the same. My CM185 is converted to 12v and has dimming issues, because I'm still running the stock 6v rectifier on it, it's getting chopped soon so I haven't bothered changing that part out yet, but works well enough as it is for now(battery hasn't ever died).

Quick part list:
RS Dual bridge rectifier
12v bulbs
15A fuse
12v battery or capactior
12v flasher if running blinkers

If you have any further questions about it, or need electrical help when hooking it all up feel free to ask.

[Bigvin]

Can anyone help me with this project? I have put CB 125 engine in a Honda CR80 rolling chassis for my daughter. The bike is great in many ways, but has been a bit unreliable due to the electrics. I was about to tear my hair out before I found your post. When I initially wired it up and got it going, I was measuring 20-30 volts across the battery. I have fried two dry state batteries and boiled dry one lead acid battery. I replaced the rectifier with a rectifier/regulator, which was fine for while, although it ran very hot. So hot in fact that you couldn't touch it, and the plastic protector melded and dripped off. Now it is not charging, so the battery goes flat. So I replaced the rectifier/regulator with what turns out to be just a rectifier off ebay. Now reading 20-30 volts across the battery again. I do wish people would described their stuff accurately!! So I am very keen to convert the system as someone on hear suggests. The wiring system I have used is as shown above. Hope you can see it when I hit post. My questions are: Would this rectifier do, see ebay item 121680145216? What is the spec of the capacitor I would need in terms of volts and microF. How is the voltage regulated? Or doesn't it matter? even though I will be getting 20-30 volts in the system?

[Elitoh7one]

I know this is an old post and there are unanswered questions. I will be attempting this on a different motorcycle. I will post some pics up when its complete. @Bigvin are you using both a Capacitor and a battery?

[Elitoh7one]

Well, I ended up getting this rectifier from Amazon Seems to work but I don't have a battery hooked up yet since I wanted to make sure it was going to work at all. The problem I am having and it might be due to not having a battery or cap, is my lights keep blowing out. I am getting 20-30v dc output though. I might be in the same boat as Bigvin.

[mamaxwell]

6v to 12v Problems
Okay guys, first response here. I am a newbie and have tinkered on a 75 cb125s for the past year. It has run fairly well but never great and I am finally trying to get it strong.
Two days ago I dove into the 6v to 12v conversion and have come up with a slew of weird problems.
So far I have added: 12v LED blinkers, 12v LED headlight, 12v LED Flasher Module and a 12v battery. I have a new rectifier but have yet to add it until I can figure out my current problems.

1. Blinkers dont blink - When I hit my blinkers (either left or right) all my blinkers light but do not blink. My 6v blinker confirmation does not light at all.

2. Only left blinkers blink - After I started tinkering with the blinker confirmation light in the speedometer I decided, randomly, to test the ground and touch the sides of the bulb socket to the side of the speedometer. When I do this my left blinkers work great AND my confirmation light starts working! BUT... when i try my right blinkers they work as well but headlight starts blinking as well!?!

3. Blowing my main fuse - after playing around with option 2 my fuse started blowing.

Any thoughts?

[vwrabbit]

I'm currently working on converting a '75 CB125 also. Double check all of your connections in the headlight. And I assume you are checking w/the bike running?

I've also heard that you should connect the white output from the stator to the yellow to help with battery charging. Can anyone confirm/deny? I also have all bulbs converted to LED, LED flasher module. Still running the 6v headlight at the moment though.