'79 cb750k Battery keeps dying!!

[JMasters0011]

Hey everyone, Can someone help me out!!?? I bought a '79 cb750k a couple weeks ago. replaced the battery, but when i let it idle, for about 30 minutes, the battery completely dies and i have no power.  I have to wait hours to try to start it again.   What would be the cause of this???  This might be an easy question...but i'm new to the motorcycle game

Thanks Everyone.

Joe

[shinyribs]

Hello Joe,welcome to the forum...and bikesYou hear alot about charging issues on these bikes. Your best bet would be to test your charging system first.It is not terribly difficult. To find out the best way to test it,go up to the top right hand corner of the home page for this site and use the search bar:




just type in what ever you need to look up.You can try"charging issues,wont charge...". You should be able to find several articles to help you out. Good luck!

[Lon]

I have a 79K also.  Have had it for a little over 3 years.  Had the same problem.  Took some time to figure it out, but ya have to test one item at a time. . Mine ended up being the regulator. . . it's a long story.  But since I fixed it, has been great and no problems.  If you find out it's the Stator, I have one.  

[sgtslag]

One issue is that you let it idle for a looonnnggg time:  30 minutes...  When the engine is turning less than 2,000 RPM, the alternator will not put out enough energy to charge the battery; the ignition system, as well as everything else, will run off the battery, draining it (starting the engine will drain the battery of a large amount of stored charge; then the headlight, alone, will draw ~4.6 Amps, plus the ignition system firing the pistons, and you have a heavy drain on the battery, which gains nothing from the Alternator at 1,000 RPM -- normal idling speed).

Visit the Home page of this site, and download the FSM (Factory Service Manual) for testing procedures for the Rectifier/Regulator, as well as the Alternator.  Cheers!

[Re-run]

Right here, these bikes do not charge much at low rpm, not enough to keep the battery charged up. A solid state regulator/rectifier can help a little bit but you would still run down the battery at idle.

[shinyribs]

Yeah,they definitely dont charge much at idle.You can see the headlight dim when it drops to around 1-1200 rpm. I puttered around in town one day at low rpms for a few hours running errands.Probably never went over 2-2500 rpms much at all.I was shooting for low engine speeds and max fuel economy...I about killed my battery.Good thing I had a kick start!Plus,air cooled motors really dont like to sit still and run for long periods of time.Best to turn even the slighest fan on them.