1975 CB125S- Headlight/Signal Dim & Shutoff

[youngman_oldbike]

Hey there guys,

Great to see a community for these old Hondas -- I'm absolutely crazy about old hondas, old BMWs and all old british bikes.

I have a 1975 CB125S, pretty good condition -- 75,000 on it. My first bike and have had a lot of fun repainting its detail work and polishing up the chrome on it. I know a little about mechanics, but not really enough to do much. I was really surprised at how much can be done if you just read the instruction manuel. That being said, it should be established I'm a student -- and plan on being a student for at least another 5 years -- so money is tight and I'd prefer to do tweaks to the bike myself instead of taking it to a mechanic.

My problem is this:

Often, if I dont warm the bike up by holding the throttle at high revs for a good minute or two it will immediately shut off when I come to a stop and put on my turn signal. Of course, it's always good to warm up a bike before you start it -- but it seems to be getting worse. Additionally, the turn signals as well as the headlight are really dim. So dim in fact, that I wont take my full motorcycle license on my bike as I dont trust the instructor to not get on me for it. When I'm stopped and my headlight is on, and I rev the engine, the headlight gets significantly brighter and then goes back to its dimness when I'm idling.

I'm kind of puzzled by the first problem as I thought I only used the electronics when I started the engine -- is this different with these old hondas? My battery is relatively new or so the past owner of the bike told me (that was a year ago now). Could it be something as simple as needing a new battery? I charged it from dead about a month ago but the lights seem to have always been dim - it's only recently, since I got it tuned up for the season with a mechanic, did it start dying with the turn signal on. Do I just need a new alternator?

Of course, if its one of the two answers I've already thought of (bad battery, bad alternator) then I'll have to spend the money -- but could it be something else? Oxidation on a critical contact? Which contact would this be?

I love this bike and its my primary mode of transportation so your help would be much appreciated,

All the best,

Liam Sarsfield

[kustommusic]

Liam, Welcome to the 305 forum. Please read and enjoy and hopfully you will pick up some good information. The first thing you need to know is we have a seperate thread for "other Honda twins" your inquiry should be there.
I personally am not familier with you model of Honda so mabey somebody else will be more help and I hope they will speak up. It does sound typical of the 305 charging system problems though. The #1 think to check is your battery connections,then all other connections related to the charging system. Check the connections to the alternator, ignition switch,and about any other you can see. #2 It might require a new diode, it regulates the alternator and is under the seat. At least on a 305 it is. If you don't get any other help P.M. me for some other ideas. Good Luck, Kustommusic

[klp]

I have several of these bikes - your dim lights are probably caused by either a weak battery or faulty ground somehwere. I would check and clean the headlight ground as well as each one for any other light that is dim.

If it tends to idle down and die you should check your valve clearances. If they are ok then I would make sure your float level is right then replace or clean your points and make sure timing is spot on. These bikes are dead simple and once set up run great.

KP