I wanted a brighter headlight on my '78 Yamaha AG175 so I installed a modern 6v LED bulb. After a few attempts I now have a nice bright headlight with high and low beam. I ended up having to run power directly off the battery and put a new switch in to cut the power when the headlight isn't needed. One bonus of doing it this way is that I can turn on the headlight when the bike isn't running. One danger is that you could run your battery down but so far in my usage that hasn't been an issue.

Supplies

6v LED bulb
Rotary tool or files
Super glue
Wire and wire connectors
A (waterproof) switch

Step 1: Buy a 6v Bulb That Has the Correct Fitting

My bike uses a P15D/H6M type 6v bulb. There are a bunch of 6v bulbs out there (and many non-6v be careful) but I chose this one because it seemed to be the brightest (1500LM) and cheapest (about $15 from Temu).

Step 2: Make Bulb Fit

The body of my bulb was too wide for the stock headlamp (your's may not be) so I used a rotary tool to widen it. Once it was wide enough the bulb mount actually came loose from the glass unfortunately. I used super glue to glue it back on and it seems fine. I live on a farm so my bike takes a beating on bad farm roads and there hasn't been any issues with the glue holding. Note: I used files at first but it's tricky to get a file in there without smashing the glass and it wasn't working that well. Rotary tool is the way to go if you can get your hands on one.

Step 3: Split Power Off Battery

When I initially tried using the bulb with the stock setup the bulb kept blowing. That was either due to voltage spikes or a grounding issue. I spoke to an auto-electrician and after looking at the bike he suggested running power directly off the battery. I used a little splitter that was in the headlight wiring but had only one wire going to it. Note: My bike has a modern AGM battery. Not sure how this setup would work with a traditional battery.

Step 4: Add a Kill Switch

Because I ran power directly off the battery I had to add a kill switch to cut the power when the headlight wasn't needed otherwise it would be on whether the bike was running or not. I bought a cheap switch that probably won't last, but is working fine so far. In the future I'll probably replace it with a waterproof switch made for motorcycle accessories.

Step 5: Run Power to the Headlight Switch

In my bike this blue wire was responsible for powering the headlight. I unplugged it and connected it to the wire running from the switch. In this configuration the headlight high/low switch still works but you turn the light on and off with the kill switch.

Step 6: Test Headlight During the Day!

This is a simple warning. Learn from my mistakes. Test your new headlight during the day for a couple of hours before you consider using it at night. In my earlier attempts I had the light go out on an unlit road and it was like someone covered my eyes with a blindfold. Pretty scary stuff. Test during the day and bring a spare standard bulb as back up just in case.