Topic: CB750 Charging Questions

[shmub]

My son got stranded on his bike today due to a loss of power; he has a '77 CB750.  As it turns out, the battery won't hold a charge anymore. But I am wondering if the charging system didn't have something to do with the battery failing.

After I determined the battery was defective, I did a couple simple tests.  With the battery out of the bike, I attached the charger and started it up.  It ran fine, and I noticed the headlight would get brighter when I revved the motor.  This would tell me the charging system is working (or at least doing something).  However, when I shut the charger off, the engine would die.  Shouldn't the charging system supply enough juice to keep the bike running without the battery?

I plan to break out the multi-meter and run some better tests, but I am curious what you guys have to say about the current circumstances.  Any advice would be appreciated.

Thanks in advance,

Dave

[Pinhead]

There has to be either a battery or capacitor across the battery terminals in order for the bike to run. You can't do any type of testing without a good battery, though..

[MCRider]

The CB750 has an alternator with an externally excited field. That is current must be entering the alternator from some source other than the alternator itsef in order for it to run. Like the battery or  a capacitor which is magic to me.

It does not have a permanent magnet in the alternator but rather an electromagnet.  That is why you cannot bump start a CB750 with a dead battery.

And it will run poorly on a weak battery more so than other models. I may not have it perfectly technically correct, but I know what I mean and I'm pretty close.

THe CB750 is fairly unique in this.

[burmashave]

If it's any help, there are sets of charging system troubleshooting procedures in the Electrics FAQ. They'd help you test the system component by component.

[shmub]

Thanks guys. That would explain my results; I will chalk it up to a bad battery.

Dave