Mystery Charging Issue

[danieljespersen]

I have had my '79 CB750F Since August. I have had difficulty with it repeatedly dying on me. I replaced the stator, the rotor, and the battery (maintenance free), and i currently get about 14 volts at 5,000 rpm. However i'm a college student and use this bike to get around. While i try to keep her in the higher rpm range, it isnt always possible. I dont think i should have to have her moving that fast or that high RPM just to keep it from dying..?
The bike died on me again the other day. I have a Battery tender that i have hooked her up to to get her going again, but i think there may be a charging problem or a drain. When i bought the bike the previous owner said that the Solenoid will get stuck sometimes, would this suck the battery dry? It may also be the regulator/rectifier, which is the only other part i could think of replacing.

Any ideas on the solution to my problem other than hooking up the battery tender every couple of days?

Any Suggestions would be appreciated.
Happy riding/building :)

[danieljespersen]

Also, when i test the battery voltage, i get 14 volts at 5-6k RPM. That seems like it is too high, am i wrong?

[shinyribs]

No,that's not too high. A healthy charging system will put out 14.5-14.7 volts.

The bike will not charge at idle,but it does not need to run 5k rpm to charge either. About 2500-300 should have it charging at full output. You may just have a case of a battery not wanting to hold a charge.Or dirty connections. Even if the bike is charging like a champ it'll never do any good if it can't push that voltage to the battery.

Since you seem to be getting good output voltage I'd start by making sure all your connections are clean and solid. If you don't find anything suspect there then my guess would be the battery.

If it is the serviceable type battery it may need water added to it.      Good luck!

*edit*- Oops.I see your battery is maintenance free.

[LukeM]

Well, you've dealt with the rotor, stator and battery.  Only one more thing in the charging loop: the regulator/rectifier. I agree with the Shiny Ribbed one: you won't see significant charging current at idle.  It starts to roll in about 2000rpm, and should peak around 14-14.5 at 5000 to 6000 rpm. If the regulator were not working well, you'd get way more than that at 5k.  I remember mine dying, and I was seeing 15-18v at the battery.  Almost cooked it.  Once I changed it out with a loaner, it runs just fine.

You might also check the diodes in the rectifier.  There's a test in the FSM.  You'll need an ohm meter to check the diodes: you should see very low resistance one way, and very high resistance when the leads are reversed.  

Make sure your ground connections and battery connections are clean and tight as well.  Also, check to see if something might be grounding out on your wiring harness.

I hope this helps.  Keep us in the loop: maybe we can help some more.
Luke M

[danieljespersen]

Yeah i know i should be getting 14 volts or so, but i didnt think i needed to be at 5000+rpm in order to achieve it. Maybe the regulator/rectifier is not allowing enough charge through? the battery is maintenance free as i said, and new. I will go through and check all the connections on the bike, and possibly for a parasitic draw. I have a battery tender hooked up to it but would love to avoid having to charge my bike nightly.

I still should be charging when i'm getting 13 or so volts while under 5k rpm, yes? just not charging as much?

This college student is learning motorcycles 101 :)

Thank you for all your responses! i'm learning a ton.

[shinyribs]

I know on my 76 model that mine is charging at full power by 3k rpm. Of course,it's a SOHC so it is a different system. If you look at your meter while it's running you'll see when it starts to charge and how much compared to idle.

I understand your battery is new,but did you fill it yourself? Or charge it yourself? If it gets overcharged when it is filled for the first time it can greatly shorten it's life span. And new batteries sitting on a shelf for a long period of time are dying the whole time. I'm not saying this is your case,just another option to consider while sorting out your mysteryNot all new parts are good