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Charging system

brettp

CB750 Junkie
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I tested my charging system a little bit last night and this morning, and I'm curious what the experts' thoughts are. Yellow to yellow wire testing on the stator while running is 10 volts at 3000 RPM, for a total of 30 volts at 3000 RPM. At 3000 RPM the bike voltage is 13 volts. It doesn't reach 14 volts until 5000 RPM. Does that sound within spec to everybody else? Emmanuel says it should be at 14 volts at 3000 RPM.
In addition, my resistance reading between the green wire and any yellow wire is 400 ohms. The manual says 2000 minimum. I'm not sure if I have a rectifier regulator problem, stator problem, or both.
 
No expert here but I think your numbers may be low.
When I want to check charging system on my bike, I keep it simple and check right at the battery posts (not digging deep if I don’t have to) your bike is charging properly you will see true value of generated power by your charging system which should be a little over 14V at 1K rpm (at least this is on my bike).
 
I thought it was low, also. The question now is, is it a stator issue or R/R issue. The yellow wires are pushing 10V at 3k RPM, so I suspect the R/R.
 
To add some context...
Revving the bike to 3k results in 13V charging. Revving to 5-6k results in 14V charging. Does this mean the R/R is working normally? It does seem to send 14V to the battery, but the motor has to be revving higher than it is supposed to. This points me to the stator, but the yellow wires push 10V, spec is 8-14V. I don't want to replace both the stator and R/R if I don't have to....
 
Checked battery tonight. 12.53V. started bike, dropped to 11.95. revved the bike to 3k and held it for a minute. Voltage got to 13.5. came off throttle, battery dropped to 11.93 after a couple minutes idling. Need new battery?
 
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I hope you have some fans blowing at that motor if you doing testing for longer period while bike is stationary!?
I don’t think your batter is bad, you just have headlight ON while bike is running and charging system is not keeping up with headlight draw. To verify this unplug the headlight and see where you are then.
 
I hope you have some fans blowing at that motor if you doing testing for longer period while bike is stationary!?
I don’t think your batter is bad, you just have headlight ON while bike is running and charging system is not keeping up with headlight draw. To verify this unplug the headlight and see where you are then.
LED headlight, taillight, and blinkers. Amperage demand is very low
 
Oh ok, I thought you are running OG setup,
Disregard what I said about headlights BUT with all LED you should definitely have higher Voltage numbers due to low draw!
 
I did order a new rotor. $215. Didn't want to take a chance on flea bay, bought new. I have been reading those go bad yet still pass the resistance tests. My R/R is preventing the charge from exceeding 14V, so I'm assuming that is ok. I couldn't find a date on the battery, so I'm guessing if it can't stay above 12V, it is a problem. 🤷
 
I'll buy a battery tomorrow and throw it in. When the new rotor comes, throw that in. I do have to wonder if my misfire is related to my low voltage. It makes sense to me, anyway. So, maybe I'll fix both things with these two replacements.
 
LED headlight, taillight, and blinkers. Amperage demand is very low
This isn't always guaranteed unless you buy your LED lights from legitimate sources. An Amazon-special headlight that my dad ordered for our 1980 CB750K claimed it was for motorcycles, but then blew both 10 amp and 20 amp fuses when installed in the bike. I took it out within minutes of discovering that.
 
I've never had to do more than check the voltage on the battery with any bike. If the bike is charging the voltage should go up to 14 or so when you rev it. If that's happening the bike is charging. With the bike off, the standing charge should be 12 and change.

The times I've had to replace regulator rectifiers, which is a common problem, the voltage didn't change no matter how hard I revved it. Another zero charge situation was with the first CB750F I had. It didn't charge at all because the kid I got it from had dropped it and flexed the case enough to crack the stator.

When I've had low charging numbers the rotor was the culprit, but that was on bikes where the rotor was a permanent magnet. The rotor loses its magnetism over time.

If memory serves DOHC CB750s have an active magnet in the rotor. Someone with more knowledge could fill in whether the active rotor loses effectiveness over time.
 
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