Motorcycle Repair: 1980 CB750k electrical, surface charge, headlight switch

Question
I'm having some electrical troubles with a bike that I just bought. it's a 1980 CB750k. A couple days after I got it, I noticed the headlight was not on. I checked the bulb, and it was good. After going through all of the connections and cleaning the switch, I was able to get it working. I was a bit concerned that it would need a jump, since I had it turned on the whole time I was tracing wires so I could check voltage. Unfortunately, it was at about 3 AM, so I could not start the bike, since it was in my garage right under my kids' room. So later in the afternoon I fired it up, and it started right away. about 2 hours later, I tried to start it so I could go to work, and it didn't have enough juice to turn the engine over. It had made it 9 hours between putting everything back together to when I started it, and in the next 2 hours, something suddenly drained the battery. the only thing I had done since starting it was to bolt on the seat. When it didn't start, I jumped it (the car was turned off, so as not to fry the system). The bike would start just fine, but when I would try to let it idle it would sputter and die, and not even click when I tried to restart it without being hooked up to the car. Prior to messing with the headlight, it had been running great, and started up easily every time. I have wiring diagrams for it, but I'm just not sure where to start. What do you think would be the most likely culprits? Thanks in advance, and sorry for being so long-winded.

Stu

Answer
Stu... WHEW! Okay, let's see what we can do about this.

First... CHARGE THE BATTERY FULLY! Check the battery afterwards to see that it is holding a full charge and not a surface charge. Most m/c shops have a load tester to do this check. Once the battery is a known value, we can go further.

The headlight switch is a dual function unit. Button out in normal riding mode.. HEADLIGHT ON. Button IN to start.. Headlights go out, starter circuit is engaged. Should also turn out the running lights up front, as I recall. Maybe that is only when the high beams are on. Switches do fail internally, leaving you with either a headlight and no starter or the reverse. Make sure that someone hasn't put some kind of high-draw headlight bulb in place of the OEM standard one. That can overcome the output of the charging system even when it is working perfectly.

Once the battery is fully charged, check the current draw with the key OFF. Disconnect the battery lead and put an ammeter in the circuit. Should show little or no draw.. few milliamps, perhaps. If it has a large draw with the key OFF, then disconnect the rectifier/regulator and check again.

Check the battery voltage with the engine running. Should go up to around 14v at 3-4k rpm. If slow or low output, check the rotor brushes and the rotor slip rings for resistance. If there is a big variation between HOT and COLD readings on the rotor sliprings, then the rotor is kaput! Rather common on the DOHC series machines with this charging system.

GO to www.electrexusa.com for a free download of the troubleshooting steps and also as a source for replacement parts.

Bill Silver