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79 DOHC CB750 charging issues

Hippie459MN

CB750 Addict
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Location
Saint Francis, MN
So last night I was out riding and I had shut my bike off and about 10 minutes later I went to start it and it was acting like the battery was going dead so I figured I would let it sit a bit and after about 30 minutes it fired right up so on my way I went. Well, it was running really odd (some very faint backfiring when I let off the gas out of 2 cylinders) and then it started to miss like it was running out of gas and finally was running on only 2 cylinders and then finally died on me only to try the starter and got nothing but clicking and every time it got weaker until it stopped all together. So I call for help and a friend came out and we hooked up jumper cables and it fired right up so we let it charge for a bit and I was on my way with him following and about half way home from there I had to stop again and let it charge up on the jumper cables again and then finally made it home. With that, I did some searching and went for the simplest first and that was check for voltage at the battery running (after I changed the battery) and I got nothing. No change and was at about 12.5 volts starting and very slowly dropping. So I pulled the stator cover to check the rotor and this is what I got...

It might be hard to see in the video but it fluctuates pretty good. Not sure if it should be a constant reading or not but either way, its way under spec so looks like a new one will be in order.


Was fluctuating between 1.5 and 0.6 ohms so I am safe to assume that would be my problem. Do you think I should check anything beyond that as well or just start there and hope for the best? I hate to throw parts at it that are not needed.

Next question, anyone know where I can buy a new one? Dennis Kirk carries them (http://bit.ly/1cCVKiP) but are out of stock (Stinks too as they are only 25ish miles from my house) and a factory Honda one is crazy expensive!
 
And one other question, How can I find out from the engine number if it is an F or a K motor? The frame is a rigid AMEN frame so that does not tell me at all. It is registered as a 1979 CB750 so I am assuming the motor is a 1979 as well. Any way to find out by the engine numbers at all?
 
I recently bought a 1976 sohc cb750 and a week or so after buying I took off on my maiden voyage. A ride from my house to my destination is about 30 minutes. When I left my house the battery was a little weak, tried to elec start her a few times until power wasn't available and decided to kick it. Started right up and I took off. Everything was going fine until about 25 minutes into the 30min trip. Started to miss every once ina while. Then started missing very badly like 2 or 3 cylinders every time I would crank the throttle. So about 200yrds from my destination is a stop sign. Stopped, tried to take off and....shit! Dead. So I waited at few minutes kicked it again. fired up but throttle made her die. So after a very embarrassing tow a whopping 200yrds. I charge her battery for an hour or so and off I went again. Charging the battery seemed to be the problem.

Next ride, I had the same problem rode for awhile bike heated up then took a serious shit. Everything I've read or seen vids, blogs, or posts on here have been saying the charging systems are junk by this time in their life.

So in short, I've pulled her in and replaced the rectifier/regulator because it seems like the best option to the outdated junk it was designed with. Upon inspection of my wiring harness two of the 3 phase yellow wires needed attention plus a red solenoid wire was fried. My stator tested good but I'm prepared to replace it too with a good stator I already have off a donor motor because why the hell not hit all angles of the problem why your on the case. At least that's my attitude.

I thought that It was a one quick fix originally but when I got into it these charging systems are very complex. For me to have to fix leads in a wiring harness plugs because of corrosion, heat damage, break down is pretty hardcore. Hopefully I've fixed my problem but who the hell knows. I'm curious as to what you find. Hopefully this may have helped you in any way. I'd like to know what you find.


Also check this out. May answer your motor questions.
http://www.hondachopper.com/engine/engine_timeline/timeline.html
 
Check the brushes on the stator. I had a similar problem and once I looked I could see one of them had broke in half and the other was worn down. It's a $10 fix.
 
I have a '75. Thought the stator was shot, changed it, still nothing. Measured fine, but figured it should be changed. That wasn't the issue. So now i have a spare. Not sure if a 75 is compatible with a 79 (I'm thinking no), but we can talk if it is.

Thank you AMPRAMPSTAMP, I. Should've looked into that. Instead, figured I'd take to a mechanic who specializes and get ALL the bugs out.

Thanks guys.
 
Turned out the alternator rotor went bad. I have a new one but with it being only like 1 degree F here at the moment, I haven't really touched the bike at all this winter. Hopefully I will get that fixed and my tins all painted here by the end of march.
 
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