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Honda cb125 stalls with lights on

5K views 17 replies 4 participants last post by  WintrSol 
#1 ·
Hi everyone,

Wondering if you can provide me some guidance...

I have a 1975 Honda cb125s. It is a 6 volt bike, and also important to note is that it has an on/off switch for the headlight (old school stuff, right?).

I've discovered that when riding with the headlight off and not using blinkers, the bike runs fine. However, as soon as I use the blinkers and/or turn on the headlight, the bike bogs down and stalls.

I got my multimeter out. With the bike idle, voltage starts around 7.20 and steadily climbs.

When I rev it with no lights on/in use, voltage jumps up and tops off around 8.30.

However, as soon as I turn on the headlight, use blinkers and brakes, etc., voltage drops to around 5.80, and the bike stalls out. Also, none of the blinker lights flash, they just remain solid.

Not being an expert, but also wanting to save some money if I can, might this be a "simple" fix, such as: replace light bulbs and retest? Replace regulator rectifier and retest? Stator, etc.? Might these things just be worthwhile to replace anyways?

Any insight would be greatly appreciated, thanks so much!
 
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Discussion starter · #8 ·
Have you checked that there is still water in the battery? A flooded 6V lead-acid battery starts to boil above about 7.2V. Take it to a shop to be tested; I expect it is fried.
I did check, and the fill level is RIGHT on the high line. Too much?

If the battery isn't overfilled and it tests fine at the shop, what's the next most likely culprit? Other posts and information I've read point at regulator, stator, the whole harness in general, bad bulbs, etc.
 
Discussion starter · #10 ·
Do you mean the actual posts on the battery, or the cable ends that clamp to it? Try removing those cable ends and cleaning them until you have bright surfaces, both on the cable ends where they contact the battery, and the battery posts too. Then when you measure, push the probes into the seam between the terminals and battery, if you can.
I mean the actual posts on the battery. I did just get back from testing the battery at the bike shop, and they said that the battery looks great.

They recommended to start cleaning connections, etc.--so I'll do that, including the cable ends and battery posts, and work my way through different connections, then retest.

Any other thoughts in the meantime would be greatly appreciated.

Thanks all!
 
Discussion starter · #13 ·
Yep, cleaning all terminals is the best thing. Those 6V systems are more sensitive to Voltage drops than 12V systems.

BTW, the wiring diagram I found has no regulator; it would be nice to have one.
Yeah, I'm working my way through, but so far nothing looks corroded or grimy, etc.

I did find what looks to be a regulator, so maybe previous owner(s) one put in...

Another discovery I made is that with the headlight off, the blinkers blink--but as soon as I turn on the headlight, blinkers go solid.
 
Discussion starter · #16 ·
Electrical stuff is my weak point when it comes to bikes, so, I caved and brought the bike to a shop. They replaced a relay, so now the blinkers blink correctly...but apparently that's all the shop did, because the bike still loses voltage (RPM won't go over 4,000 when any lights are being used) and stalls when you are using too many lights (headlight, blinkers, brakes, high beam, etc.). Needless to say, that's a big problem when the sun starts going down.

I will re-test to see if voltage continuously climbs when bike is idle and no lights are being used.
 
Discussion starter · #17 ·
So a couple of quick updates:

As it currently stands, when the bike is sitting idle with no lights on, it hovers around 6.15 volts. And when I rev the bike, voltage tops out at 6.22 and does not go any higher...so it seems that going up to 8 volts (and beyond) is no longer happening. Hopefully that holds true as I continue to test.

But, if I use any lights, voltage drops below 6 volts and bike stalls.

Being a bit annoyed that the bike shop didn't seem to address the main problem of voltage dropping and bike stalling when lights are used, I did start digging into the wiring a little more bravely...and after taking much of the bike apart and taking the harness half out, I found that multiple wires are frayed, cut, poorly spliced, and are literally hanging on to plugs and connectors by just a couple strands in multiple spots.

I'm not an expert, but this can't be good, and I'm hoping that is is the main cause of the voltage drop. Either way, I'll be working on cleaning it all up, redoing plugs, etc.
 
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