'82 GPz750 Electrical Issue - Charging

  • redrkt
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'82 GPz750 Electrical Issue - Charging

24 Mar 2025 10:53
#910455
Hi folks,

I've run into an electrical problem with my 1982 GPz750.  Short summary: engine starts and runs well, but after running for about 5 mins or so, shutting off the engine and then restarting, the battery can barely push the starter. Do it again (run for a few more minutes), shut it off, and then battery can't get it started.  Battery, measured by a multimeter at the battery, shows 12.7V when turned off, While it's running, I'm measuring 13.75V to 14.4V at the battery. When I recharge the battery, the bike starts up well (starter motor turns quickly).

I replaced the regulator/rectifier recently because it was only showing a 12.9V.  I've cleaned all contacts except those in the headlight (that's next), but not sure of the cause. The battery is new last year, electrolyte levels are good.  I would think that with the regulator/rectifier providing good power to the battery post-start, the battery wouldn't have any issues.

Any advice please?

Thanks,

Jeff

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  • TheTrout
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Re: '82 GPz750 Electrical Issue - Charging

24 Mar 2025 11:02 - 24 Mar 2025 11:03
#910456
How old is the battery?  I've seen plenty of batteries with voltage, but no amperage left.  The only way to know would be to have the battery load tested.
 
1978 KZ1000B2
Last edit: 24 Mar 2025 11:03 by TheTrout.

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  • Wookie58
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Re: '82 GPz750 Electrical Issue - Charging

24 Mar 2025 11:06 - 24 Mar 2025 12:33
#910458
When you re-start what is the battery voltage "when cranking" the engine (anything much below 10v would suggest the battery is goosed) Also connect a voltmeter black to battery neg and red to the body of the starter (anything more than 0.3v when cranking would suggest a bad ground)
see page 4 of the below thread for an explanation of why batteries fail to deliver current
LTD 1100 Project - KZRider Forum - KZRider, KZ, Z1 & Z Motorcycle Enthusiast's Forum
1982 KZ1000 Ltd
www.kzrider.com/forum/11-projects/617631...-82-begins?start=192
kzrider.com/filebase-alias?view=download...d-fault-diagnosis&ca
Last edit: 24 Mar 2025 12:33 by Wookie58.

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Re: '82 GPz750 Electrical Issue - Charging

24 Mar 2025 12:45
#910472
redrkt wrote:
Hi folks,

I've run into an electrical problem with my 1982 GPz750.  Short summary: engine starts and runs well, but after running for about 5 mins or so, shutting off the engine and then restarting, the battery can barely push the starter. Do it again (run for a few more minutes), shut it off, and then battery can't get it started.  Battery, measured by a multimeter at the battery, shows 12.7V when turned off, While it's running, I'm measuring 13.75V to 14.4V at the battery. When I recharge the battery, the bike starts up well (starter motor turns quickly).

I replaced the regulator/rectifier recently because it was only showing a 12.9V.  I've cleaned all contacts except those in the headlight (that's next), but not sure of the cause. The battery is new last year, electrolyte levels are good.  I would think that with the regulator/rectifier providing good power to the battery post-start, the battery wouldn't have any issues.

Any advice please?

Thanks,

Jeff
Something seems off.  If you are getting 14.4 VDC across the battery when running, presumably, with the engine reved up, then the charging system is good.  That points to a battery problem.
Ed
Carb O-ring Kits : www.kzrider.com/forum/3-carburetor/61807...-o-ring-kits?start=0

www.kzrider.com/forum/faq-wiki/618026-new-owner-things-to-know

1981 KZ750E2
www.kzrider.com/forum/11-projects/604901...z750e-project-thread
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Re: '82 GPz750 Electrical Issue - Charging

Today 17:26
#910530
Thank you all for your great advice! I had thought about the battery, but it's relatively new (since last October)...good to know that a battery can come out the store defective.  Makes sense, voltage ok, but no depth. I'll remember that :)

I bought this bike new, had it since I was in my early 20's and still ride it, of course. What a great bike - I enjoy the wrenching as much as riding.

By the way, Wookie58, loved the description of how batteries fail.

I really appreciate your help.
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