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'81 CB750c Not charging. Help!?

Garrett411

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Hey Ya'll

First post on here as i am working on a friends 1981 CB750c for him.

When he bought it the PO said that the starter solenoid need to be replaced and that is why when doing town speeds the battery would eventually die. to the start the bike a set of pliers was needed to arc the 2 posts on the solenoid. So then we replaced the solenoid and the new one came with a 15 Amp fuse which burnt right away, replaced that with a 40 and the bike now will start with the switch. At that time we also replaced the battery. Rode the bike around a few times and eventually would die because of a drained battery. I then took the stator cover off and tested the two rotor strips (ground on battery disconnected) with the switch on and off testing the strips alternatively with the multimeter. Each time it measured around 4-6 ohms at first and the would climb higher after a while. This from what i have read is the kind of thing i should be experiencing with these measurements. So i concluded that i must be the rectifier/regulator. With the bike idling the battery would stay about the same reading 12V about, running about quarter throttle to try and get a charge it would only go up a few hundredths of a volt which doesn't seem like enough. I was expecting around 13'ish. So we replaced the old and im assuming stock regulator/rectifier with a new one. We also replaced the flasher with a new one that i had laying around. The bike will run for a 20 mile highway drive, maybe more starting with 12-12.5 volts, maybe a bit more. But once we get back to town and if we hit traffic or stop lights it sometimes will die and need to be jumped and charged.
Long story short im just wondering what ya'll think of this? I personally just built an XS650 cafe and have no experience with these Cb's. Thinking the problem could still reside in the rotor? Brushes maybe? Any advice is appreciated!
 
No manual. I kind of have gone through the basics, will do all over tomorrow just to double check as specified by manual.
 
The new starter solenoid would burn out a 30 Amp normal fuse. The original solenoid that was faulty had a flat lead type which said it was a 30 Amp and that wasn't burnt when i replaced it and looked it over. We looked around for a while and had heard of people using up to 50 amp fuses on these generic starter solenoid.
 
Ok just am tinkering on it with a little bit of down time today and retested the rotor with my multimeter.

Red on inside ring: .9-4.1 ohms then would periodically jump to a much higher number.Ex: 36 ohms.

Black on inside wring: .5 - 4 ohms. Some case as the first with it hovering there for a while then climbing.

The rotor seems to have a little what appears to be welds were i suspect it arc'd on the outside edge. The 1st two pictures show these little tacks. The second 2 just show the rings of the rotor and the inside of the stator surface. Let me know of what you guys think. 1.jpg2.jpg3.jpg4.jpg
 
I suppose if it was me I'd start by checking voltage at the yellow wires from the stator with the bike running. Between any two of those wires should be... I believe... around 50 volts AC. At least that the way it is on my 1100 Kaw. If you're getting correct voltage from the stator, it would point to the reg/rectifier of bad connections maybe.
 
Okay so with bike idling and the stator unhooked we tried the each of the yellow wires and the black for the ground and on each the voltage measured less that a volt AC (.3-.8). Even when the bike was running about 1/4 throttle the voltage really didn't go up. So this is all pointing to a faulty rotor and/or stator?
 
If you're getting basically nothing from those 3 wires, that should indicate as you said... bad stator or rotor.

Check resistance on those same wires and that should tell you if the stator is bad. The manual section shows stator continuity should be .41-.51 ohms. Check for that between each pair of wires. Forgive me if this is obvious, but be sure to check the resistance from simply touching the meter leads together and subtract that from the actual reading. When I checked mine I had .3-.4 just by touching the meter less together, and forgot to subtract that... my stator was bad but I didn't realize it because the reading was so close, but then it hit me that I basically had a short in my stator. Anyway, it sounds like you know how to use the meter better than I do anyway, but I thought I'd mention that.

So, if the stator reads good, that leaves the rotor as the suspect. Could be the brushes I suppose.


NOTICE: I edited the "total reading" reference to say "subtract" instead of "add" for the correct reading.
 
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