This last summer riding season I encountered 2 gremlins on the 1972 Suzuki GT750J. I hope this forum submission will help others, should they encounter similar problems.
On a sunny July saturday, while waiting at an intersection for the light to turn green, the engine quit?cold turkey. No lights, no starter, no ignition, nut?n! I jiggled the bike around, switched the ignition key off and on and suddenly everything worked again. On with the ride! But in hindsight, I should have headed home to figure out what was going on. As it was, I carried on and the bike quit again. I rolled the bike to a nearby parking lot and called Andrea to come and get me. Once home I hooked up the trailer to my truck and went out to get the bike.
Studying the GT750 electrical diagram, it was clear that such a sudden failure is most likely because of a fault in power feed from the battery to the ignition switch. Or possibly the power feed after the ignition switch.
As a start I pulled the main (and only) fuse for an inspection. The fuse appeared intact and my multi meter indicated no resistance between the ends. But when I went to re-install it, one of the metal end caps fell off, revealing a corroded solder joint. Aha I thought?problem found. New fuse installed and this time I made it to the bottom of the driveway before everything quit again.
Unfortunately (or maybe fortunately!) The bike did not go again and I was home. I tracked the fault down to a corroded/burned up red wire 2.8mm connector pin in the 4-pin ignition switch connector in the headlight shell.
With these Hitachi connectors (that are common in most 1970?s Japanese bikes), you can remove each pin from the connector block by depressing the little retaining clip. There are special tools for this but I use a tiny screwdriver. The corroded connector came out in two pieces. I carefully cut the crimp clips on the old connector and was able to crimp a new connector in its place on the same wire.
I buy my connectors from Vintage Connections in California (
https://www.vintageconnections.com/). They have a good website and ship all over the world. Furthermore, they pick up the phone when you call, and they know what they?re talking about. Their connectors are solid brass; not plated pot metal like too many others found on many websites.
With everything connected again, the ignition on and the bike fired right up. I went for a nice long ride with no issues or problems. However, I got to thinking; this wiring system carries all the current to power the bike; headlight, taillight, ignition and turn signals. (I always ride with the headlight on?better chance of being seen!) On low beam, the headlight which is rated at 45/55W, will draw close to 4 amps. Add 2-3 amps for the ignition system and ~1 amp for the tail lights-turn signals, makes a total current of 7-8 amps.
I wondered, what would be the impact of the voltage ?drop? in the wiring that carries all this current? The regulator controls voltage based on what it ?sees? downstream of all the loads. In other words, the regulator would try to make up for any voltage drop in the primary power feeder circuit. And, I should be able to see this by monitoring battery voltage while riding.
The next step, I strapped a little Koso volt indicator to the handlebar, hay-wired it to the battery and went for a ride. The voltage readings confirmed my fears:
-Resting voltage with engine off was 12.9V. Good enough!
-Idling and riding around town with headlight off, the voltage ranged from 13.9-14.6 volts. A wider range than on my modern bikes but given the primitive
voltage regulator, probably acceptable.
-with the headlight on, the voltage climbed to 16.1V.
-16.8V on high beam!
-I was right. Yikes!
Definitely some fixing was needed. If I did nothing, I feared that eventually the smoke that runs the electrical system would come out and this time in the middle of B*^^-F#*^ nowhere. Other ideas that I considered include re-wiring with heavier gauge wire, find an LED headlight, or ride with the headlight off. What I landed on was to run the headlight feed directly from the battery, controlled by a relay that?s switched by the existing bike headlight wiring. (Relay switching currents are very low)
Fortunately, there?s no need to faff around building one?s own relay system for the headlight?this has been done by a Canadian motorcyclist who lives in Japan. His name is Jim Davis and his webstore is called Eastern Beaver (
https://www.easternbeaver.com/motorcycle-wiring-kits/). His parcels raised eyebrows at my house until I reminded Andrea that the guy is Canadian and the beaver is our national animal. I had no idea what she was on about?really! The Eastern Beaver H4 headlight wiring kit is plug and play. It includes male and female H4 plugs that connect to the headlight unit and bike socket, wired up to two Panasonic micro relays for high and low beam. It all fits in the headlight shell.
Installation took about an hour. Now without the headlight current passing through the red wire the bike voltage runs in the 13.9-14.6V range. Not perfect, but a lot better than 16V. QED
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