Motorcycle Repair: 750K Charging...or not, regulator rectifier, carbon brushes

Question
Bill,
We have an '82 CB750K that we don't think is charging correctly. The battery isn't strong enough to start it after a hour or so ride. The bike will run if compression started but won't turn over enough to start. After putting the battery on the charger for an hour or two, things are fine, and it fires right up. Is there a way to test components while still on the bike(the parts that is, not riders)? I can see where the rectifiers(which I think are the culprits in this case) plug into and where the harness from the stator etc from the engine connect. Can you tell me what the color codes of the wires I need to connect my meter to are, and what the readings should be. I'd really appreciate it.
Thanks,
Jeff & Sue Robison

Answer
Jeff, I don't have wiring diagrams for the DOHC machines, but a lot of troubleshooting can be done on the bike. You can go to www.electrexusa.com for a LOT of info on charging systems, however.

The system consists of stator (usually yellow wires)whose wires are interlinked electrically, but should not show a path to ground. Not a common failure source, however.

The rotor has windings and two tracks for the carbon brushes to ride on. Rotors have a BAD reputation for failure, usually when the engine is HOT. Check the resistance values COLD and HOT on the rotor to see if it is going OPEN or out of range when the parts get hot. In rare cases, the key shears off and the rotor spins on the end of the crankshaft. Usually they are very hard to remove and require a special removal tool to do the job.

There are a couple of carbon brushes mounted in holders inside the left crankshaft cover. If they are worn down and not contacting the rotor slip rings, then you have a no charge condition.

The regulator rectifier is one unit, I think. You need a factory book to check the unit and sometimes standard VOM meters are not sufficient to accurately test the unit. Usually if everything else checks out good, then you can change the rectifier/regulator unit to complete the repair.

Wiring and wire connectors are always a factor on older bikes, so check everything carefully for cleanliness and tight connections.

Hope this steers you in the right direction....

Bill Silver