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Old 04-19-2012, 01:50 AM   #1
cbguy
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Charging system on 1981 cb750k

Ok i was hoping someone here could help me out with this, I have a new batt and a new rectifier, but its not holding charge if i charge the batt it will start and stay on but once i turn it off is completely dead again. Can anyone steer me in the right direction. Any kind of info would help.

Thanks guys i really want to start riding my bike soon and would love to make a nice cafe out of it soon!!
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Old 04-19-2012, 03:28 AM   #2
Patrick
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Do you have a volt meter?
How many volts are there at idoe and at 4000 rpm?
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Old 04-19-2012, 01:49 PM   #3
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Patrick ill get you that info.
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Old 04-19-2012, 07:02 PM   #4
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ok so i bought a charger and multimeter so i could give you those number but iv had it charging for an hour and nothing is coming on.

Dam another great day outside and no bike!!!
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Old 04-19-2012, 07:08 PM   #5
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When mine went out I replaced the stater/ generator also. Didn't want to spend the extra $$, but it was worth it.
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Old 04-19-2012, 07:23 PM   #6
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i went thru and back with my old cb almost wanted to set it on fire at one point. but yes, we need more info and would like to know mechanically/electronically inclined you are to be able to help.
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Old 04-26-2012, 08:20 PM   #7
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Ok Patrick and mx6er.......here is what i have.

When the bike is off and try to start it there is no juice what so ever, if i jump the bike it will start right away and stay on but as soon as the i turn it off again it looses all current.

I bought a meter and did some research the batt is holding charge when its not connected to the bike, as soon as i connect it and try to start the bike it something drains the batt.

I have tested the rotor and it is showing 5.1 ohms also checked the brushes and arent worn out.
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Old 04-27-2012, 06:35 AM   #8
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Even though it's a new battery it sounds like it may be bad.
If you can bring the battery by I can load test it for you, no charge.
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Old 04-28-2012, 08:33 PM   #9
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I went through and back trying to fiqure out a 82 CB900C. The worst part is buying a new part, say a stator. And then you come to realize that it's a p.o.s just like one I just took out. The trouble I always had was degradation of the charging output as the engine heated up. The conclusion I came to is some battery's have excessive internal resistance and if the charging system is barely adequate the battery will not stay charged. Those bikes were notorious for weak charging systems. You might need to check the ac voltage coming out of the stator, your field coil could be bad or it may be bad connections or shorted wires. The wires that handle the load are to small too which causes voltage drop and can be helped by adding an additional ground and positive straight of the rectifier.
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