Topic: 1980 CB750F DOHC Charging Issue / Out of Ideas

[mrmach5]

 Sorry , I know this is the SOHC forum , but was not getting much help on the DOHC site so hoping to get some ideas that I've not thought of.
 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. Oh and a brand new Duralast Gold AGM battery
I'm at a loss at this point, not sure what direction to go.

[scottly]

Try replacing the brushes.

[bryanj]

And pull hard on all the wires at connectors.

You could try putting a voltmeter on the brush connections to read input into rotor, no input =reg or wiring. Input all time= brushes, rotor or stator wiring

[Flyin900]

Try replacing the brushes.

That is a head scratcher for sure. I used to specialize in the DOHC motors years ago and I have never seen that happen. The rotors do fail more than a stator IMO and experience. I would also be checking the wiring and connectors all the way through your harness over to the compete regulator, stator and associated system parts for any shorts, poor connections at the pins in the connectors.

As Scotty noted the brushes may be suspect too, although I have never had those fail there's always a first time on 40+ year old parts.

Also the 5 ohm rotor reading is borderline, as the FSM spec is 3.6 to 4.4 ohms, with 7-8 ohms out of range for sure. Aftermarket rotors are definitely suspect these days, yet if you tried a Honda used rotor that was in spec I would trust that reading for sure over aftermarket. Electrosport had poor quality stuff IMO years ago and were not a brand to use. That could have changed now though.
I suspect you have something going on beyond the rotor with all the parts required. The change from working when cold to the heat build up failure is normally a failing rotor sign, which changes output as the motor and rotor get hot.

[HondaMan]

I'd vote for the connectors outside the rotor area. When they oxidize (which happened some 40 years ago) they will conduct when cold, then heat up and barely touch one another, the hotter they get. Clean the connectors (all over the bike), as this is the #1 thing I find troubles in on all of the Hondas made from 1979 until the 1990s. There are many of those tiny blade-style connectors in the designs that are running at 100% of their rating, which makes them heat up, then oxidize.

When you've cleaned them all (I use steel wool or ScotchBrite pads) I can most-assuredly advise adding a drop of LPS-1 (not LPS-2, 2.5, or 3, just 1) to each connector: you'll never had to do it again and they will stop oxidizing.

[scottly]

The two 1979 CBXs in this picture both had charging system failures on the same day, in early July 1979. Both bikes had about 8000 miles on them. The silver bike had intermittent problems for a while, and Chris had replaced the regulator and it seemed OK, but on the 2nd day of our ride to Colorado, while in the middle of nowhere, Nevada, it quit charging. He pulled out the headlight fuse to lower the draw on the battery, and we continued, hoping to make it to some semblance of civilization before the battery died, but after only a few miles, the red bike quit charging. We managed to limp into Winnemucca, where I had cousins, and we found the outer brushes on both bikes were worn out. We had to get some brushes from an electric motor shop and file them down until they fit into the Honda brush holders.
Another thing that can happen, especially when a rotor has a "shorted" or lower resistance, is the brush springs can get over-heated, losing their tension.

[mrmach5]

Thanks all for the input,
so with a new regulator, and new stator, all that wiring and connectors are brand new. Last week I installed a new ignition switch as well trying to eliminate another possible  culprit.  Although the bike only has 6K miles on it and the brush wear line is still in sight, I will order a new set of brushes this morning and give that a try. Like I said out of ideas so just trying to eliminate possibilities .
Thanks again

[MauiK3]

I bought a vintage connector kit to allow me to rebuild any connectors. It's easy.

[bryanj]

You need to start logical testing
1 is the rotor getting power all the time
2 iif 1 is yes is the stator producing ac on 3 phases all the time.
3 if 1 and 2 ok is the rectifier getting the ac
4 is the rectifier giving out dc