Will a weak stator ruin a battery?

[Bmb2492]

82 750 nighthawk with 4,500 miles on it. It hasn't been tagged since 03. I've put 2 batteries on it. One just kept going dead. I checked the rotor and it's sitting on 5.2 ohms. The stator tested fine but they both looked rusty. I cleaned them up. I've only rode this bike 10 miles since I got it. I took the battery to get it charged and it came back with 11.5 volts and I still had to jump it off. Revving it to 5,000 rpm, it doesn't go over 13 volts. I checked the brushes and they aren't worn but I have to scratch and poke to get a reading. This bike has sat for 20 years until I got it last month.
Someone has had the stator off before because the gasket is gone and it was rusty inside. Is the stator the problem? Maybe the brushes? I just don't understand why I have these charging problems with a bike with such low miles... I'm lost!!!!

 

[SenseAmidMadness]

Yep, repeatedly draining a battery to almost dead or completely dead can be damaging. Draining a battery to its breaking point can even be fatal to its function. It's not a guarantee (I've been lucky a couple times on a previous bike when I left heated grips on) but you could have a second killed battery on your hands. Failing to make a full 14 volts absolutely means there's something wrong with your charging system.

Somewhere on this forum there's a link to the Electrical chapter of the CB900F/C shop manual; I forget exactly where but you should be able to search for it. That chapter gives you a full diagnostic procedure (multimeter required) for all the parts of the ignition and charging systems. It's much better than the corresponding chapter in the 750's shop manual. It would be a very good idea to test all your charging parts and compare them to the spec sheet in that chapter. The stator and rotor both have several pass/fail measurements.

The "scratch and poke" line is a possible clue though. Perhaps you could, gently, take a fine-tooth file or some 400-grit sandpaper to the end of the brushes to make better contact like people do to ignition points. Also look at the surface on the rotor that they contact -- is it shiny and do you get good and immediate readings between the two poles for the brushes?

Charging problems don't usually happen based on mileage unless you're just wearing the brushes out; corrosion is the silent killer of old motorcycle electrics. The rust inside that stator cover and across all your other electrical parts certainly won't help anything.

 

[Bmb2492]

Thanks for the help. I am really irritated because the bike was soo well kept and never rode but I am coming to see that a bike sitting forever is much harder on it than riding it every day. I got stranded on the side of the road yesterday. When I got the bike home, I turned the key on and it immediately went from 10 to 6 volts. I'm reading a lot about how the rotors can test good but be faulty once heated.

 

[SenseAmidMadness]

...I am coming to see that a bike sitting forever is much harder on it than riding it every day.

Very, very true for pretty much any kind of vehicle. I say this to everyone who's looking to buy anything with wheels: maintenance is far more important than mileage.
A garage queen motorcycle that has no miles on it very rarely has its issues noticed and corrected, and a ton of parts (everything made of rubber and every fluid) have expiration dates that may be postponed but can never be removed. In contrast, a bike that's been ridden 100,000 miles has had plenty of attention to get that far, from a caring owner who either made sure it was all done himself or took it to a decent mechanic very often. For the same price I'd pick the high mileage machine with a full stack of maintenance records every time. It will usually end up being cheaper to own, too, because you don't have to spend a small fortune on parts as soon as you buy it.

If you've got a good thick pair of leather gloves (welding gloves are ideal) and an electric impact gun with the right socket to get the cover bolts, you could run the bike for a while and test the rotor and stator while they're hot. Just gotta be careful.

 

[Bmb2492]

I think I just need to step away for a couple days. I've worked on it everyday for a month to get it this far. These are the before and after pics.

 

[Old_Crow]

These bikes like to run. I just bought what I would class as a garage queen. Perfectly kept 10th anniversary edition with only 20k miles on it.
A bit different this one, because it also came with a stack of service records that indicate oil changes and services yearly, regardless of miles ridden. Bike started and ran good, but as I start to put miles on it (I bought it to ride, don't even own a garage) it's beginning to run better with every tank of gas I put through it.
Take a break, go fishing for a day, drink some beer, relax, and come back to the bike with a new set of eyes. The battery that you took somewhere to get charged is either bad, or the guy's battery charger wasn't working right. After charging you should have well more than 11v in the battery. I'd start by testing the battery and getting it charged right, then I'd check for a draw in the electrical system of the bike. If there was no draw, then I'd start looking at the charging system. I understand that these bikes don't make over 12v unless the rpm is well above idle (3-4,000 rpm maybe?) Like S.A.M. said, find a service manual.

 

[RattyRatbikes]

where can i get a stator for cb750k? for the life of me i cant find them except 200 dollar ones on ebay which i dont trust.

 

[brettp]

Got my stator from Ricksmotorsportselectrics.com. it was $200 but I paid that to make sure it wasn't trash.