Hi Guys,

Bought a modified K100rs a couple of months ago and when I bought it I had it checked over by a mechanic and he said it was fine. I took it on a long ride and it died. I was pulling off onto a slip road and pulled the clutch and that was it. It would just whir and not start. I found if I left it for over 20 mins it would restart but then the lights on the speedo would dim and it would die again and not start. It all seems to point to a battery issue so I had the battery replaced but on the next ride it would die again any time I pulled in the clutch and the revs dropped. I managed to get it to a garage but on the way there it was bucking and misfiring. 

We tested the battery when I got there and it was low but jump-started it and when it was revved the battery did charge  but when I was riding it the revs would have been high and it still died. I had bought a Hall sensor after reading posts here but it seems like a charging issue. The spark plugs were pretty black when they were taken out. The only thing I did read was that as the speedo has been changed there might be a resister or a bulb to kick the alternator into charging when idling that might not be there now? There is also another connector on the alternator which the mechanic isn't sure about? 

Any help would be great as I just want to ride my bike and at the moment its spent its life in the garage

Which brand of speedometer is it?

It's this one 

https://cafe4racer.eu/en/universal-digital-speedometers/96-motorcycle-dual-odometer-speedometer-gauge-led.html

If anyone has any ideas on how to get the fuel gauge to work as well that would be great. 

Thanks

Have you checked the regulator/ rectifier on the rear of the alternator? I think your brushes are worn below the minimum length.

I'm pretty sure it's the lack of the bulb creating the resistance for the alternator to kick in. Does anyone have a easy ish fix for this?

I had a similar problem with dying after a run when I pulled in the clutch.  Replacing the voltage regulator and brushes on the alternator fixed it.  I suspect that a bad voltage regulator won't compensate for the dropping rpm fast enough to keep the ignition running.

You mention "whirring" of the starter.  That sounds like a problem with the sprag clutch.

Thanks for all your help guys, I can't quite work out why it was dying when I was actually riding it though. It was bucking and misfiring on the way to the garage maybe lots of things have decided to go wrong at once?

Bfgben wrote: It was bucking and misfiring on the way to the garage maybe lots of things have decided to go wrong at once?

Maybe, as .75 and Stumpy have indicated, but if the alternator isn't charging in the first place because the previous owner didn't provide for a circuit to excite the alternator into charging the system after the OEM instrument cluster was replaced, then as the battery loses power, bucking and misfiring from lack of current to run the electronics can occur.

A full inspection of the alternator—replacing parts where necessary—should be considered plus a test of its output at various rpm.

Custom speedometers pose a significant challenge for many Brick owners and likely have scuttled as many projects as they have enhanced. Marulab's BEP interface was developed to make such conversions somewhat less challenging.

Put a voltmeter on it.  When the alternator kicks in it should be in the low-mid 13s at idle and then go up to 14-14.2 when you rev it up above 2,000+ RPM.

If you want to emulate the charge light from the OEM cluster then put a small 12V bulb between the blue wire coming from the alternator and any green/black wire.

duck wrote:put a small 12V bulb

1.2 watts or 2.0 watts power rating. No bigger.

The factory indicator bulbs are 3W. 

https://www.realoem.com/bmw/enUS/showparts?id=0572-USA---K569-BMW-K%2075%20S%20(0563,0572)&diagId=62_0534

Just curious: What happens if you use a higher watt bulb?

duck wrote:The factory indicator bulbs are 3W. 

https://www.realoem.com/bmw/enUS/showparts?id=0572-USA---K569-BMW-K%2075%20S%20(0563,0572)&diagId=62_0534

Just curious: What happens if you use a higher watt bulb?

I suspect that a higher watt bulb will work fine for the charging aspect, but there would not be enough current going through it to light up when charging is low or not happening.

Bill

Even without an indicator bulb the alternator will self energize from residual magnetism at 3,000rpm or thereabouts.  You can tell it has energized when the voltage on the B+ terminal goes from the battery voltaage to something like 13.4 or higher.  You will also see the headlight get noticeably brighter.

It's entirely possible to ride quite a while without the alternator energizing, especially in low rpm city traffic situations.  Without the bulb you need to rev the engine to get it charging.

If there's no alternator output you are running off of the battery.  The alternator  is putting out a decent amount of power at 13.4V or whatever the idle voltage is.

duck wrote:The factory indicator bulbs are 3W.

Thanks Duck. Idiot here quoted Guzzi specs...

If the moto hasn't been idle for so long that there is no residual magnetism, if the alternator is able to generate current by one means of excitement or another, if its output is sufficient to charge a battery then the moto's battery is likely faulty making it unable to hold a charge long enough for useful riding.

The alternator's condition and output need inspection; the battery needs to be load tested. The battery's connections, the alternator's connections, all ground connections and all electrical connectors should be inspected for tightness and corrosion. That might be unsettling news for Bfgben but those are tasks that he should be able to undertake with little difficulty. In fact from what I've read, some folks seem to prefer working on their motos rather than riding them; Bfgben might discover he's in that cohort.

It's all part of the pleasure of ownership.

indian036 wrote:

duck wrote:The factory indicator bulbs are 3W. 

https://www.realoem.com/bmw/enUS/showparts?id=0572-USA---K569-BMW-K%2075%20S%20(0563,0572)&diagId=62_0534

Just curious: What happens if you use a higher watt bulb?

I suspect that a higher watt bulb will work fine for the charging aspect, but there would not be enough current going through it to light up when charging is low or not happening.

Bill

Yes, there would be, powered by the bike's battery.  Do you know how that bulb illuminates?  One side of it is tapped into the 12V+ wiring of the bike. The other input is the blue monitor wire from the alternator. When the bike is turned on but not running there is 12.xV from the battery powering everything including that charge indicator bulb. The blue wire is at 0V so the bulb illuminates.  When you start the bike the blue wire from the alternator also comes up to 12+V so there's the same voltage on both sides of the bulb and it doesn't light up because the voltage potential across the bulb is effectively zero.

duck wrote:

indian036 wrote:

duck wrote:The factory indicator bulbs are 3W. 

https://www.realoem.com/bmw/enUS/showparts?id=0572-USA---K569-BMW-K%2075%20S%20(0563,0572)&diagId=62_0534

Just curious: What happens if you use a higher watt bulb?

I suspect that a higher watt bulb will work fine for the charging aspect, but there would not be enough current going through it to light up when charging is low or not happening.

Bill

Yes, there would be, powered by the bike's battery.  Do you know how that bulb illuminates?  One side of it is tapped into the 12V+ wiring of the bike. The other input is the blue monitor wire from the alternator. When the bike is turned on but not running there is 12.xV from the battery powering everything including that charge indicator bulb. The blue wire is at 0V so the bulb illuminates.  When you start the bike the blue wire from the alternator also comes up to 12+V so there's the same voltage on both sides of the bulb and it doesn't light up because the voltage potential across the bulb is effectively zero.

I'm familiar with how the bulb illuminates and why it goes out when the alternator is charging.

I'm happy to be corrected if wrong, but I believe the regulator circuitry limits the exciter current coming from the battery through the charge light. It doesn't need much in the fields to start producing electricity. When its charging, the fields are fed from the alternator output via the regulator, with variable feed to match the alternator output to the bike's needs for running and topping up the battery after starting.

If I'm correct about the exciter current limitation, that's why a higher watt bulb wouldn't light up, but I don't know the wattage threshold for not illuminating.

At least we have a regulated output in the bikes. The 1978 Mercury outboard I owned had an unregulated alternator. It usually sat at ~18V when at operating revs!

Bill

indian036 wrote:At least we have a regulated output in the bikes. The 1978 Mercury outboard I owned had an unregulated alternator. It usually sat at ~18V when at operating revs!

Yeah - and so did most 1980/90s GS-variant Suzukis. Some idiotic engineer designed the three phase regulator so that

blue/white was always regulated
red/white relied on the battery being good to pull it down
green/white was only switched in when the headlight was turned on.

If the battery was anything but spot on, the inevitable result was serious overheating, high-voltage backfeed and bye-bye alternator, regulater and rectifier. I made a small fortune fixing that problem by fitting Honda Superdream combined reg/recs. The only downside was the price of s/hand reg/rec units tripled from £5 each to £15 each when the local breakers (wreckers) for miles around realised why I was buying up all their Honda reg/rec units...