Topic: 80 CB750F DOHC Charging Issue

[mrmach5]

Sorry I know this is the SOHC forum, but not getting much input on the DOHC site.
So here is my issue,
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. 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.

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!

[Flyin900]

These bikes have an issue sometimes with the old rotors failing, the stators not so much. Many of the aftermarket rotors are not up to snuff these days, even Ricks rotors recently have been getting a bad rap on some forums. The reading on a stock good OEM rotor as you noted is well below 5 ohms and usually about 3-4 ohms IFIRC.

A used OEM rotor is an option, if you can find one that someone has spec'd with a meter on EBay showing the ring readings. Another option is a Yamaha rotor from an XJ650 if I recall the model correctly will bolt right up, yet you need to use the Honda rotor bolt and not the Yamaha rotor bolt. They have a reputation of a better build and less failure rate.

That said I used to specialize in the DOHC bikes, mostly CB900/1000C's and only ever had one failure out of over 12 bikes previously owned in that genre.

The Yamaha rotors used to be dirt cheap used vs the OEM Honda ones, yet that may have changed.

[calj737]

Youve posted a great deal anout charging, spent money on parts, but have not posted a word about your battery.

What is the resting voltage before key ON? After key ON? Charge it fully, then let it stand for 2-4 hours. Then retest it.

If the Reg/Rec is not getting accurate information from the battery, it can't send correct voltage to it, thereby causing it to deplete rather quickly. Even a "brand new" battery will go bad, especially if it has been heavily discharged. I literally just replaced a new battery this AM bought in Jan of this year because it sat in a project vehicle and discharged and desulfinated.

[mrmach5]

Ill check to see how old the battery is. I'll charge it and report back with the voltage findings.

[fizzlebottom]

These bikes have an issue sometimes with the old rotors failing, the stators not so much. Many of the aftermarket rotors are not up to snuff these days, even Ricks rotors recently have been getting a bad rap on some forums.

Man, I thought it was just me! I had a Rick's rotor fail on me after a year. Ended up finding another one in my parts pile and its been great since. Probably an old OEM Honda unit that'll last for another 10k miles.

[bryanj]

Have you checked the brush length

[commander60]

Go to CB1100F.net  Mike Nixon's charging guide , great

[mrmach5]

Sorry for the delay, a wise man once said , don't work on your motorcycles on your anniversary .
Ok so fully charged the battery was a  13.4. V , with the key on it was down to 12.2 within a minute. Was not able to tell old the battery is ( I usually date them with a sharpie) but perhaps this battery is just not able to keep up. Once the bike totally cooled off I checked the rotor again and is 5.0-5.4 ohms, right in spec.
So I'll be battery shopping and hope that puts an end to my issue.
Thanks

[calj737]

You should also start the motor and test voltage AT the battery to confirm your charging output.

[mrmach5]

That 's the thing, will a fully charged battery it was making 13.7 to 14. at 4-5k rpm but withing 15 minutes of riding the voltage was down to low 12's even at RPM.

[calj737]

That’s fine. You confirmed the stator charges, but the battery can’t hold the charge. Dead battery is my verdict. I could be mistaken…

[mrmach5]

I"ve purchased a new Duralast Gold AGM 220CCA Battery, and have it charging now. Rain coming so the test drive will probably be delayed.

I hope this solves it, however I'm gonna feel pretty goofy having chased my tail for a bit.

Thanks for all the input

[scottly]

That 's the thing, will a fully charged battery it was making 13.7 to 14. at 4-5k rpm but withing 15 minutes of riding the voltage was down to low 12's even at RPM.

That proves that your charging system isn't working! If it was, you could remove the battery and the charging system would continue to provide power as long as the RPMs were high enough to offset the demand. The battery is only needed to get the engine running, after that the charging system keeps it going.

[scottly]

These bikes have an issue sometimes with the old rotors failing, the stators not so much. Many of the aftermarket rotors are not up to snuff these days, even Ricks rotors recently have been getting a bad rap on some forums.

Man, I thought it was just me! I had a Rick's rotor fail on me after a year. Ended up finding another one in my parts pile and its been great since. Probably an old OEM Honda unit that'll last for another 10k miles.

The quality of Rick's parts came up a few years ago. Having once worked in an electric motor shop, the standard practice was to dip the stators in a thick varnish, stabilizing the windings. If you look at a stock stator you can see the varnish. It locks the windings in place: otherwise with every rotation of the rotor's magnetic field the windings would shift back and forth, fretting against each other. Rick doesn't seem to think this is necessary for either stators or rotors that are spinning at up to 9000 RPM.  

[70CB750]

That 's the thing, will a fully charged battery it was making 13.7 to 14. at 4-5k rpm but withing 15 minutes of riding the voltage was down to low 12's even at RPM.

Few years back it took me forever to track down charging issue.  The stator measured ok when cold but stopped charging when it got hot - this was SOHC 750 - and the new to me stator was bad too.

I finally went full Edward Morris and bought NOS Honda stator and never had any problems since. 

[mrmach5]

Well the test drive showed similar results , charging 13-14.3 at 4k rpm , however again after 15 minutes of riding an warmed up the charge rate goes down. Looks like I'm not done.

[scottly]

the rotor failed the test. I purchased a new rotor


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!

Your new Rick's rotor is bad. I wouldn't trust the stator, either.