1980 CB750F Charging Issues / Frustrating!!!


CB750 Enthusiast
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Spartanburg , SC
This goes back to September 3rd,

So recently, My 80 F stopped charging , I was able to get it home before it quit. After running several tests on the stator, and rotor,,,,,,, the rotor failed the test. I purchased a new rotor
Installed then tested running voltage, and looked great 13.7 - 14.4 . A week or so later took it out for a nice long ride. This time not so lucky, it quit on me a couple miles from home.
I installed a new rectifier- regulator and checked connections. This morning hooked up a temporary volt gauge to monitor it while test driving. When I first fired it up it was making 13-14 volts at RPM, after riding 6-7 miles it was down to 12.3-12.5, barely battery voltage.
So it seems whatever is happening is tempature sensitive. I'll be getting out the multi meter and start the process again. I'm not ruling out the rotor even though its brand new.
Also the brushes look great , bike only has 6,500 miles on it.

Sept 10th
Installed a new stator and tested charging and looked good. Again on the test drive I had a voltage gauge up on the dash to make note of charging voltage at different RPMs . At a steady 4k it was making just shy of 14v. About 15 minutes into the ride , the voltage output was dropping and eventually barely making positive voltage at 3-4k rpm, and none at idle. Made it home ok ,and back on the lift. Removed the cover and checked the rotor and measures at 7.8-7.9 ohms . The spec for these rotors is 5.0 ohms which is what it was when I installed a few weeks ago. I know that rotors that test below 5 ohms are considered bad, what about ones that test higher? With the new stator having no real effect on the issue, I'm highly suspicious of the rotor . Just because its new does not mean it can't be bad.

Oct 26th
So still not fixed . Originally the Rotor tested bad so I replaced with a new Electromotive. Suspected it was bad so they replaced it with another new unit. Same result , charges fine cold but once up to temperature it stops making acceptable voltage. I then tried a used Honda rotor that tested good. Same result.
So far I have replaced the rotor 3 times, the stator once and the regulator -rectifier twice , all with the same result.
I'm at a loss at this point, not sure what direction to go.
I would suggest looking at the wires of these parts for damage. Maybe you have something shorting, a corroded connector, etc.
My rotor wasn't performing well and I replaced it with $200 one to make sure it wasn't a Chinese knock off.
Yeah, I would start resistance testing terminals and wires between components. You may be making the correct voltage but losing a good amount of total current to heat, which can toast regulators and rectifiers and stators very quickly.
Highly agree. You have new wiring but you can also have bad crimp(s) on new wiring which will also create heat and high resistance. Keep in mind one strand in the whole cable will give you “good continuity” result, proper test for a cable is actually voltage drop. For example if you have 10ga wire and you hook up headlight bulb to the known good battery and bulb is dim, you know something is going on with that wire…!
The battery is a brand new Duralast Gold AGM . As far as the other components, I've had the factory Rec-Reg , as well as an aftermarket. no change. Same with the Stator, factory and aftermarket no change. The original rotor tested bad, so it was replaced with a new Electrosport rotor, still not charging once hot.
Surmising that rotor might have been bad it was replaced with another new Electrosport unit , same result. I then purchased a used Honda rotor that tests good , same result.
I have a new Ignition switch on the way, as of the regulator is not getting a reliable 12 volts then the power being sent to the brushes is diminished and might not be fully exciting the eletro magnet. One of the tests I've done is holding a 0.15 feeler gauge with the key on to check for magnetism . This bike fails this test.
Perhaps I need some schooling here. But my understanding of R/Rs is they take AC current and convert it to DC, feeding that to the battery. Why would the R/R need 12V?
Secondly, the brushes are taking energy from the stator, not being fed energy.
Lastly, I have never heard of the magnetism test. What is that?
Well brettp , with all due respect , your understanding of this charging system is way off.
With key on, the regulator sends 12+ volts to the rotor via the brushes. The spinning magnetized rotor creates electrical energy thru the windings of the stator, and sends this AC current to the rectifier via the three yellow wires , then converts to DC current to the Regulator and battery.

I know you were just trying to be helpful, but when you don't understand the subject, better to just not post than put out misinformation.
This will help
The rotor and stator purpose I understand. I do know my limitations on the mechanism of them. If you look back you'll see I wasn't trying to help at all; I was asking questions. Specifically about those things you explained, of which I admit to limited knowledge.
Oh, and if we're going to get snippy, I'd like to point out the brushes aren't on the rotor, they're on the stator.
Not trying to be snippy at all, but once again you are way off base. The brushes have nothing to do with the stator and are not even connected . The brushes are mounted inside the side cover in the brush holder. They are spring loaded to make contact with the two contact rings on the rotor. The brushes connect with the regulator via the White and Black wires. The Stator has three yellow wires that connect to the rectifier.