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80 CB750F Not Charging when warm

mrmach5

CB750 Enthusiast
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Spartanburg , SC
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.
 
Well, crap. If you have a new rotor and RR, I think the only thing left is the stator and/or connections in that circuit. Look at all the connectors for melting plastic (meaning, open them up and look closely).
 
Just tested a few things, the rotor is at 5.8 ohms should be 5.0ohms when cold ( still a little warm) I'll test again when stone cold. All three yellow wires have continuity between all three, and no short to ground. Connectors all look good. Brush holder and brushes look good. The stator being the only component not yet replaced, thats kinda where my mind goes.
 
Just tested a few things, the rotor is at 5.8 ohms should be 5.0ohms when cold ( still a little warm) I'll test again when stone cold. All three yellow wires have continuity between all three, and no short to ground. Connectors all look good. Brush holder and brushes look good. The stator being the only component not yet replaced, thats kinda where my mind goes.
Yeah, that's what I'm thinking, too. After all of that, what else is left?
 
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.
Frustrating!
 
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.
Frustrating!
Yes, rotors can be bad out of the box. I've read that on here, anyway, and that's where I'm getting it. It's not from experience. In any case, I replaced my rotor recently, paid 200 +40 shipping. Was worth it to have a quality rotor. But it still hurt.
 
Brettp,
where did you source your rotor from? There are tons of Chinese ones avaialble , then there is Ricks, not sure where his come from?
 
Does anybody know? If VB43A carburetor will work on a 1979 CB750 , Super SportF. It is calling for.vb43b
 
Does anybody know? If VB43A carburetor will work on a 1979 CB750 , Super SportF. It is calling for.vb43b
Alek, did you post this here as a question pertaining to the original subject line? I think your question needs to be a new thread outside of this one.
 
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