BMW K bikes (Bricks)

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1Back to top Go down    HELP-losing power while riding on Mon Aug 22, 2016 3:06 pm


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Since I am new to this I was hoping i can get some info regarding the issue Im having.
I started recently having issues with my 1989 k100.
First it was with the battery loosing power. I charged it and it worked fine. After couple of days riding it started loosing power and the bike would stop while riding. Took it to a mechanic and it seemed that the battery totally lost power, as soon as i would connect jump cables the bike would start and work fine.
Today it happened the same thing while i was riding i noticed the engine wasnt sounding right and soon enough it stopped and it wouldnt start up. I used the jump cables again and it worked fine.
Im starting to believe that the bike is draining the battery somehow or maybe just the battery is gone bad. 
Anyone had the same issues or is just me....
Any feedback is appreciated 
Thank you in advance


2Back to top Go down    age of battery? on Mon Aug 22, 2016 7:34 pm


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Is the battery fairly new and fresh, or has it been through a few seasons and likely to be on it's last legs?

A fresh battery at 100% state of charge should read 12.7 vdc or higher with no loads pulling current, aka open circuit voltage or OCV.  If it is reading 12.0 or lower OCV then it is at about 45% state of charge.  There are tables on the internet, so measure you battery voltage and determine it's state of charge.  Then try to charge it and see if it holds a charge.  A weak, old, or worn out battery can't hold a charge anymore, and that is likely your problem.


3Back to top Go down    Re: HELP-losing power while riding on Mon Aug 22, 2016 11:34 pm


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Take the battery to an auto parts store and ask them to load test it. It's free. (Because it helps them sell new batteries.)

Current stable:
86 Custom K100 (standard fairing, K75 Belly pan, Ceramic chromed engine covers, paralever)
K75 Frankenbrick (Paralever, K11 front end, hybrid ABS, K1100RS fairing, radial tires)
86 K75C Turbo w/ paralever
94 K1100RS
93 K1100LT (x2)
91 K1
93 K75S (K11 front end)
91 K75S (K1 front end)
14 Yamaha WR250R

4Back to top Go down    Re: HELP-losing power while riding on Tue Aug 23, 2016 5:11 am


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5Back to top Go down    Re: HELP-losing power while riding on Sat Oct 08, 2016 9:49 pm


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Suggest you check the 'Kill' switch for intermittent operation.
(Corrosion in my K75 Kill switch will prevent starting occasionally and will also cause the engine to cut out suddenly.)
I have found that if I pull the end of the switch backward it will stop killing the bike, so I have a wad of paper behind the end of the switch.



Silver member
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From the sound of things your battery is okay, what may not be working is the charging system.  After the battery is charged it's all good, after some riding the bike konks out.

Put a volt meter across the battery and start the engine.  One the engine is running and revved up to 3000 RPM the voltage should rise up read at least 13.2 volts, preferably closer to 13.8, if it goes up any higher than 14.5 volts you have a problem, overcharging, although this won't cause the bike to quit.  My guess is that you'll see the voltage drop after the engine has been started and not come back up.  The wheels stop when the voltage gets below 11.5 volts on an old K bike.

Once up and running the bike gets just about all it's power from the charging system, if that's not working your range will be quite limited.

- John

7Back to top Go down    Re: HELP-losing power while riding on Sun Oct 16, 2016 10:24 pm


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Life time member
I agree that the charge from the alternator sounds to be the problem and probably due to a faulty voltage regulator or brushes. VR and brushes are in one unit and easily replaced.

An alternator will start to output max voltage at around 1600rpm so only necessary to rev the engine to ≈2000rpm to measure the charge voltage across the battery. A battery won't start to charge till approx 13.6V from the alt. At around 14.4V a lead acid battery will start to gas.
Ideally the charge voltage should be somewhere around 14.0V ±0.2V.

If the charge voltage is down then the charge light should be glowing. If its not then the battery may be kaput but from what you've described I'd say the battery is done anyway as it doesn’t seem to be able to hold a charge.

At 12.45V the battery is at 75% charge. Below this the battery is considered to be past its serviceable life. If the static voltage reading (ign off and measuring across the bat after a charge and a resting period of at least six hours) is less that 12.45.... the bat can't hold a charge and is dud.

If the voltage remains above 12.45V then test under load. Turn your lights on for 10 mins. Load voltage will go down to <10V but the important thing to note is how it recovers back to above 12.45V
A fully charged battery at around 13.0V should recover to >12.45V

1988 K100RT     VIN No.  0094680
1989 K100RT     VIN No.  0097367 (naked)  
1996 K1100RS   VIN No.  0451808
     Fuel:  95 Octane
Engine Oil: Nulon Full Synthetic 15W50
Gear Box Oil:  Nulon Synthetic 75W90

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