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Honda 125 electrics

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Alan Donovan28/03/2021 17:24:10
61 forum posts
29 photos

Hello all.

I am coming to the latter stages of restoring a 1975 Honda SL125.

The original indicators were beyond being salvaged and have been replaced with aftermarket parts. All electrical connections have been made. The bike has a 6 volt battery and 6 volt electrical system as per original design.

The bike has a new battery and it is fully charged. With the battery in this condition the indicators work as I would expect. Bright lights with a respectable flash rate. With the battery half charged the lights appear a little 'sluggish' to illuminate and the flash rate seems slower. Also, in both scenarios, the neutral indicator light dims when the indicator flashes. I would expect this to be normal performance - but I am happy to be corrected on this.

This is where it goes further astray.

The problem is when I switch on the headlight this seems to 'suck' the power from the battery such that the indicators are feeble to none-existent. The indicator circuit is getting something as the the headlight dims in unison with the indicator relay. I assume this is not normal performance and that the indicators and headlamps should perform together as needed.

May I ask for suggestions as to what may be the cause of this, and how it may be corrected.

I have not check this out with the engine running - if that is relevant.

Apologies for the 'long ramble' but I am trying to give as much information to you as possible.

Many thanks in advance to anybody who responds.

Best regards. Alan.

SillyOldDuffer28/03/2021 17:36:02
8490 forum posts
1891 photos

Doesn't sound completely wrong, but I suspect an unwanted high resistance in the circuit. Does the bike use earth return electrics? That is one battery terminal strapped to the frame, with one wire to each device, which is also bolted to the frame to complete the cicuit. If so, I suspect a bad contact to the frame somewhere due to corrosion or looseness. Try undoing the frame connections, giving them a good clean, and then tightening them up firmly. Could be as simple as a dirty or loose battery clamp.


Spurry28/03/2021 18:04:11
220 forum posts
72 photos

I think it certainly worth a try going through your scenario but with the engine running, before delving too deeply into what could be wrong.


Samsaranda28/03/2021 18:06:31
1399 forum posts
5 photos

I doubt the battery can run all those lights at full output without the engine running, probably would be a different story if 12 volt. Dave W

Tim Stevens28/03/2021 18:15:41
1587 forum posts

Small motorcycles including the 125 Honda range suffered from the poor electric kit that was 'standard'. I have improved many similar machines by removing the battery, and replacing the rectifier with a modern 4-diode assembly, and adding a 14V Zener diode as used on full-sized bikes of the period. This serves to divert excess current (if any) , and turns it to heat, so the Zener needs to be on a chunky heat sink in the breeze. With this kit, it also helps - now that they are available - to use 12v LED bulbs in the flashers, and perhaps - if you go out at night - in the side, tail, and headlamp. As long as you are sensible you can manage without a battery, but do use the bike for several trips in broad daylight first, with the lights on, so you know the system is reliable and will work well.

Come back to me if I can help further.

Cheers, Tim

Edited By Tim Stevens on 28/03/2021 18:16:34

Maurice Taylor28/03/2021 18:25:08
211 forum posts
36 photos

Hi ,please don't waste your time looking for faults until you've run the engine and then switched everything on and checked the battery voltage and that it's charging.

I think the lights used to dim at tickover anyway on a 6volt system.


Dave Halford28/03/2021 18:35:24
2006 forum posts
23 photos

Are the new flasher bulbs the correct wattage? The 12v ones on my CB175 were 21W, the same as a car, if you check out the current drawn with everything on that poor little 6AH battery will soon need a lie down.

If memory serves the earths are via black wires on Hondas. I would check the resistances back to the battery and then run the engine like it's supposed to be, the voltage will go up, the amp load will go down and the genny will supply the rest.

If it still flashes the head light - then you have a problem and not before.

Pete Rimmer28/03/2021 18:37:08
1219 forum posts
63 photos

The 6v Hondas had very marginal electrics and one might even have had to run the bike during MOT to get them to pass on those items (there is a facility in the testing manual for this). Also, from that era there were two different systems for the ignition. For example the OHV CG125 took it's coil power directly off the magneto so it could be run with no battery. The OHC CB100 took it's coil power from the battery however so with no (or poor) battery it would not start.

This is working from memory I hope I'm remembering it correctly.

Phil P28/03/2021 19:14:22
802 forum posts
194 photos

I run a 1972 Honda XL250, the electrics are very marginal on that as well, the bike runs perfectly until the headlamp is on then it starts to misfire at high revs.

I never got to the bottom of it and dont understand why it should do that as the lighting and ignition coils are separate units and should not affect each other.

I tried an LED bulb in my headlamp, but the rectifier must be not giving a very clean DC output and the LED lasted a matter of minutes.

I solved the problem by never going out in the dark on mine smiley

I am with the others on this, run the engine and see how it performs before you change anything else.


Edited By Phil P on 28/03/2021 19:15:22

Mike Poole28/03/2021 19:52:46
3305 forum posts
73 photos

The standard flasher relay was a bimetallic device which will vary the flash rate with the voltage, electronic plug in replacements are available which can maintain the flash rate when the voltage drops, the flashers will still be dimmer but at least flash at the right rate. 6V electrics need to be in excellent condition to be in with a chance of working satisfactorily.


old mart28/03/2021 19:59:07
3720 forum posts
233 photos
Posted by Mike Poole on 28/03/2021 19:52:46:

The standard flasher relay was a bimetallic device which will vary the flash rate with the voltage, electronic plug in replacements are available which can maintain the flash rate when the voltage drops, the flashers will still be dimmer but at least flash at the right rate. 6V electrics need to be in excellent condition to be in with a chance of working satisfactorily.


That electronic unit would work with LED flasher bulbs which use much less power.

Alan Donovan28/03/2021 20:23:45
61 forum posts
29 photos


Thank you all so much for your input, I really appreciate your help.

I will try the lights with the engine running first, as that is an easy thing to do. Regardless of the result, it may be worth checking the cleanliness of the 'earth paths' anyway, as the bike is still a 'work in progress', and I wish the bike to be in the best condition for road use.

From the comments so far I have plenty to consider, and I will be so much wiser (I hope) about motorcycle electrical systems.

All the best and thank you all.


Grindstone Cowboy28/03/2021 21:27:35
854 forum posts
64 photos

If at all possible, I'd recommend adding separate earth wires to everything.


John Olsen28/03/2021 22:44:35
1240 forum posts
94 photos
1 articles

The SL125 has the same setup as my 1971 CB100 had. The permanent magnet alternator has two windings, one which is always connected to the rectifier and one which is switched in when the headlight is turned on. This type of alternator has no regulator, and depends on the alternator acting like a constant current source at higher RPM. For that to work, the load current must be approximately equal to the design current, for each switch setting. Any excess current will overcharge the battery, while if there is a shortfall the battery will go flat. So it is important for the bulbs to be the specified wattage, no use trying to put in a more powerful headlight for instance. You can put in a better one of the same wattage. Things that are only on intermittently, like the flashers and brake light, will use stored capacity from the battery and the charge will (hopefully!) be made up during normal running. This means that there is likely to be a bit of excess charge, which is not good for the long term prospects of the battery. A shunt regulator like the Triumphs of the era used would be a good addition, not that Lucas electrics were generally any good!

But since your current problem is with the engine not running, I would be looking for high resistances in the circuit. The problem could be in the ground path, which is through the frame, or in the live connections or switches. If you have painted the frame and then screwed on the ground connection for the battery, that could be one cause for instance.

I think I still have my CB100 workshop manual somewhere, I wonder if I can find it.


Gerard O'Toole29/03/2021 07:23:34
135 forum posts
10 photos

I think your electrics are performing as designed. I remember the flashers flashing slower at tick-over if the battery wasn't great.

The fact they are working with a full battery would suggest you don't have ant earth problems either. Although i always run extra earth wires from the rear lights and headlamp shell.

Fitting led lights will affect the balance of the circuit and might result in an overcharge battery. Fitting a modern regulator would sort that. They are available for lucas 6v systems - i am not too sure for Japanese electrics.

not done it yet29/03/2021 08:38:58
6733 forum posts
20 photos

My observation would be that the battery will not last long if not kept fully charged.

My Hondas were a generation previous to the one in question - I have a dim recollection that the alternator output was controlled by increasing the excitation when lights were switched on? The rectifier would certainly be better replaced with a modern bridge rectifier. Even an AVR would not go amiss, I would think - zener diodes with huge heatsinks were fitted simply because there was no other voltage/generation control on the charging circuit.

John Olsen29/03/2021 11:03:17
1240 forum posts
94 photos
1 articles

At that time the Hondas up to about the 350 twin all had permanent magnet alternators. Some, including the 200 twin, had a regulator. The fours came out with proper excited field alternators, which gave decent regulation and a lot more available power, which didn't stop Honda providing a totally inadequate headlight on the 750 four. Earlier British bikes either had a dynamo with a regulator, which would have been fine apart from the inadequacies of the maker, or else a permanent magnet alternator, improved about 1970 by the addition of the zener regulator. My Triumph Blazer SS 250 had one of those, and also a large electrolytic capacitor so you could in theory take the battery out to save weight. That implied that you could get the engine turning over fast enough to generate enough power for the battery and coil ignition to work...I never tried that.

They didn't change the excitation, the alternator had two windings, one always connected, and the other switched in when the headlight was turned on. The alternator had six coils, one winding was two of them and the other was four. If you need to pull the alternator, take the screw out, remove the back axle from the back wheel and screw it into the large female thread on the alternator magnet casing. Do it up firmly, give the end a tap with a lead hammer, and repeat until the magnet housing comes off. This worked on every Honda of the sixties and early seventies that I ever had the chance to try it on. Saves buying a special puller.

I would check if the rectifier is selenium or silicon, if not silicon I would change it. A shunt regulator would also be a good idea, it needs to be rated for about 60 Watts. Otherwise you can cook a battery quite quickly if you are running with lights on in the daytime and the headlight bulb blows.


Tim Stevens29/03/2021 11:28:19
1587 forum posts

The Shunt Regulator which J Olsen refers to is the same thing as the Zener Diode I suggested - except that they were only made in a nominal 12v version (actually about 14v). Good luck with finding bulbs which match the values in the parts list. The scarcity, nowadays, of six volt bulbs in a variety of Watt values, is one of the reasons I suggested running at 12v.

And while the electrics may be 'running as designed' they were never much good ...

Cheers, Tim

Gerard O'Toole29/03/2021 13:48:39
135 forum posts
10 photos

Agree Tim, converting to 12V might be a good idea. It certainly makes finding bulbs etc easier. Having said that, I have two motorcycles with 6V electrics ( one a Lucas dynamo the other a Miller dynamo) and they both work ok. But i don't generally use at night and they have very simple electrical circuits, simple lights and horn. No indicators , no ignition etc.

If you do move to 12 V then you should be able to source a suitable voltage regulator/rectifier. Originally , Zenor diodes , as supplied by Lucas, were only available for Positive earth systems . No sure what is available now but the combined regulator/rectifier seems to be the recommended way to go. I have fitted one to a 12V motorcycle and it all works quite well.and has done for over 10 years

Alan Donovan29/03/2021 18:15:37
61 forum posts
29 photos

Hi All.

Thank you all for your advice and guidance.

I will check the system out with the engine running and also check out or provide new earth connections. It seems to be the general opinion that 6 volt systems are pretty poor and I should not expect too much of an improvement unless I change to 12 volt system. Thinking back to my Lambretta days (late 60s), I seem to remember that had a 6 volt system and that was not brilliant. 12 volt conversions were 'all the rage' then.

With respect to the availability of spares, I did manage to buy a number of spare bulbs and plugs of the correct specification via the on-line auction house.

Tim Stevens - you have certainly given me an alternative option and I shall not discount converting to a 12 volt system.

Best regards to all. Alan.

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