Honda 125 electrics

If you convert to 12V you may not be able to use a 60/55 watt headlight bulb as the output of the alternator may not be high enough. I would ask the question on a motorcycle forum, somebody will have the exact answer for your bike.

Smaller Quartz headlamp bulbs are available - from memory try 36W/36W, or 36/30?

One factor I failed to mention - if converting to 12v it is helpful to have insulated connections to both sides of the generating coil(s). This helps when connecting a full-wave rectifier, and when using a Zener Diode. The process involves finding the 'earth' connection for the alternator windings, separating it from the metal casing/housing, and connecting a new wire to this winding. If you have a two coil system (one always on, one for the headlamp) then you can use whichever wire you disconnect - so you don't have to make a new connection through the crankcase grommet. I suspect that you need a (pensioner) electric expert who worked on 1960-1980 motorcycles.

Regards, Tim

These alternators don't have any earth connection on either of the coils, since they use a full wave bridge rectifier. If you disconnect the alternator from the rectifier you can check for resistance to the frame, it should be open circuit. The Lucas one on my Blazer SS went short to ground, so I had to rewind it. The ones on bikes with a magneto combined with the lighting coil often do have the coils earthed at one end, but the type on the CB/CL/SL 100/125 range don't.

It is actually possible to use the existing Honda coils to charge a 12 volt battery. This will raise the engine rpm at which charging starts, approximately doubling it. If you spend a lot of time at low RPM this might not give adequate charging. I actually rewound mine at the time, with just one winding with a few more turns, and fitted a home made shunt regulator and a 12 Volt battery. That meant that I was able to fit a 55Watt H4 Hella quartz halogen light, which was much better than the original and was good for night riding. It was actually better than the light the 750 four came with at the time. So a bike capable of maybe 125 mph came with a headlight that was barely adequate for 60 or so.

A shunt regulator is a pretty simple circuit, the main thing to remember if you build one yourself or find someone to do it is that this is a high vibration environment. My first attempt worked fine for a short time, until most of the components fell off their leads. The second attempt was potted in epoxy and lasted until I sold the bike, which by then had been taken out to 125, fitted with a hot cam, and a larger carb. It was capable of 80mph and might have done more if I could have arranged higher gearing. Oh, it also had the front end out of a Suzuki 175, mainly so I got the 8 inch twin leading shoe brake, which wouldn't fit into the Honda forks.

John

Hi All.

Here is some feedback on the Honda SL125.

Checked and cleaned the earths, and ran some extra earth wires from the indicators (belt & braces). Started the SL125's engine and made the light/indicator checks with engine running.

With the engine running you are able to have both indicators and the headlamp on (as suggested by a few of you), but I did note that even with the engine running the headlamp did 'dim' a little as they came on. Also the indicator flash rate seemed OK to me. From the consensus of opinion on 'vintage Honda electrical technology' I am happy with the result.

The timing is not quite right (according to the strobe), so that is tomorrows project all being well.

Many thanks to everyone with their views and advice. Keep safe.

All the best. Alan.

On the lefthand engine side cover is there a separate round two screw cover that has a drum inside that centrifuges the oil?

If so make sure the spring loaded plate (which seals to the crank end inside the drum) attached to the post is free to move. They have have a nasty habit of nibbling into the post, sticking, then failing to seal to the crank properly.

The fact that all the lights dim before the headlamps, or flashers, come fully on is not a problem showing up - it is built in to the lights because the tungsten filaments have a cold resistance which is much lower than when hot. So, to start with, the cold headlamp bulb or flashers draw too much current (for a second or two) and there is not enough current to feed everything else at the normal value. There is no 'spare' power available in the generator or the battery. Changing to LED bulbs (except for the headlamp) would mask the problem in two ways - the LEDs would need less current, and they are less fussy about the exact voltage.

Cheers, Tim

Edited By Tim Stevens on 02/04/2021 20:20:35

Edited By Tim Stevens on 02/04/2021 20:22:26