I have read many posts of problems with intermittent stalling and I must say that my bike's issue is nowhere as ominous-sounding as some of you are dealing with. Regardless, I would like to understand what is happenings so here goes:

1987 K100RT with 160,000 miles
Cold and hot starting are not issues, in both cases the bike starts immediately.
Battery voltage is 12.6V
Fuel pump has good pressure


The bike has been very reliable over the past 18 years but just recently the bike will stall - infrequently - when slowing to an idle, such as when approaching a stop light. The engine does not faulter, it just dies. I touch of the start button and the engine immediately re-starts and idles without complaining.

I don't know if I am dealing with a fuel or electronic issue. Since the engine doesn't sound like it is starving - and only happens as I slow to a stop (never stalls or hesitates at speed) - I don't think it is a fuel delivery problem. On the other hand, since the bike immediately re-starts and idles fine, I find it difficult to think it is an electrical issue.

12.6v is too low.  I suspect you are having the same problem I had with my old K100RS.

It would stall at stops after a period of running at engine speeds of 3000rpm and up.  It seemed the problem was a slow response in the voltage regulator to the engine dropping to idle speed, allowing the electrical system drop to the loaded voltage on the battery.  This voltage was too low to keep the injectors, fuel pump, ecu, lights, etc running properly, killing the engine. 

Replacing the voltage regulator, alternator brushes, and the old battery solved the problem.  The brushes were worn, and the battery was only 3 years old.  I suspect the worn brushes and bad voltage regulator failed to keep the battery fully charged leading to it's early demise.

One of the clues to my problem was that the alternator light would come on at a stop an instant before the engine died, not after, which is the normal way it should work.

I would approach the problem in two stages.  First, check the alternator brushes and have the battery load tested.  This load test can be done on the bike, but the best results would be with the battery disconnected from the electrical system.  Most automotive parts or battery shops are able to do a battery load test.

The second stage is approached if the problem persists after the battery and brushes test good.  That entails replacing the voltage regulator.

Although I haven't mentioned it, I am assuming that all the electrical connections, particularly the grounds on the frame and engine, as well as every connection related to the battery, alternator and engine control wiring are both clean and tight.

Check the valve clearances, tight exhaust valves will cause that.

Thank BOTH of you for your comments. I’ll check alternator brushes, battery, and exhaust valves tomorrow. Hopefully the problem will be resolved by doing these things. If not, there is the voltage regulator.

I post in a couple of says with an update.
Dave

(Update) I assessed the charging system following the service manual and it checked out okay. The alternator is kicking out between 13.5v and 14v at 1500 rpm and voltage does not fluctuate when high beam and heated grips are toggled on/off. The local auto parts folks load tested the battery and said the battery was good BUT it could use a charge (their test registered 12v which is too low).

The battery holds a charge if disconnected from the bike, but the voltage dropped from 12.65v to 12.15v over a period of 12 hours when not connected to the trickle charger. The only device which should be drawing current is the clock and while I don't have the specs on the clock, I am quite sure it is not responsible for this voltage drop. The upshot of my findings is I probably have a short to ground.

A visual inspection - alas with the faring installed - resulted in no obvious grounded or damaged wiring. Next steps seem to warrant looking at continuity, voltage or excessively resistance on various wiring segments. I did pull fuses and checked for leaking voltage but came up with bizarre readings which left me with a lack of clarity which may have been because the ECU was still connected. With the ECU removed and the Neg(-) ground wire removed, there is no voltage between the Pos (+) anode and ground.

The battery is 2-1/2 years old and I have been pretty good about maintaining it and -usually- keep it on the trickle charger. Also, my driving style is certainly more puttering as opposed to canyon carving.

If the voltage is measuring 12.65 as soon as removed from the charger and drops to 12.15 after 12 hours then it is cactus.
Immediately the charger is removed the voltage should read 13 - 13.2 and drop to 12.6 - 12.7 after 12 hours of no use and when in the bike at 3000 rpm it should be 14.2V checking the voltage at 1500rpm can result in erratic readings.

Looks like I'll be shopping for a new battery...

Have you checked the shims because that stalling can be caused by tight clearances on the exhaust.

Rick, yes, sorry I forgot to mention that.

RicK G wrote:If the voltage is measuring 12.65 as soon as removed from the charger and drops to 12.15 after 12 hours then it is cactus.
Immediately the charger is removed the voltage should read 13 - 13.2 and drop to 12.6 - 12.7 after 12 hours of no use and when in the bike at 3000 rpm it should be 14.2V checking the voltage at 1500rpm can result in erratic readings.

I agree. A good battery that has been fully charged will show 12.8 volts for weeks, if not months, after being removed from the charger.

Testing the voltage regulator for alternator output at a given rpm won't show the problem.

My regulator showed proper output of 14 volts at 1500-2000rpm.  The problem on my bike was that as engine rpm dropped to idle when pulling in the clutch and closing the throttle after an extended run at 3000rpm the regulator did not increase the field current fast enough to keep the alternator's output at the proper voltage.  The problem was caused by the slow response of the regulator.

With the lights on, including the brake light, the load was enough to bring the system voltage down to where the engine died when the alternator output dropped too low.

As I mentioned, the only clue I had was that the battery light on the cluster would blink on when stopping right after pulling the clutch and just before the engine died.
 
Hope this makes sense.

My bike did that a few times, suddenly quitting at idle and low speeds.

As I  was planning to check the electrical connections anyway I cleaned the ignition switch and it

never did it gain.

I realise there are other reasons for the symptom, but it worked for me.

Bob

Bob, what is involved with cleaning the ignition switch. Do I need to disassemble it or just spray it with contact cleaner?
Dave

Re regulator: I am not excited about replacing a regulator u less I can confirm it is the culprit. I will connect a volt meter to the battery terminals and see what is happening as I ride about town.

There is a detailed procedure in the Download section on this Forum concerning the removal and cleaning of the ignition switch.

It is not a five minute job and you need to be careful when removing the switch outer piece.

The contacts will need cleaning with a fine emery paper , and coated with  Dialectic grease prior to reassembly.

Whether this is your problem or not, cleaning of all the electrical connections on any bike of  a 

certain age, is vital to long term reliability .

Good luck

Bob

I agree on keeping electrical connections ubër clean and as a general rule do a pretty thorough job. That being said, I have not disassembled the ignition switch. I’ll take a look on the forum for ‘how-to’ instructions.

A follow-up:
I replaced the battery and cleaned the ignition switch. 

To iterate, the intermittent stalling happens as I approach a stop and engine speed is typically at 2000 rpm or less (the bike has never stalled while idling). Sometime as I approach a stop I can tell the engine is just winding down but there is no firing going on AND I notice all indicators - including the CLOCK are blank. I mention this because I believe if it was a voltage regulator I would NOT lose my clock or gear selection indicator. Also, if it was a failing ignition switch, the clock would remain functioning. Also, the alternator/charging light does not flicker or come on (until such time as the electrical power comes back on at which time the alternator light, clock, headlight, etc. come on just as if I was starting it. 

After a moment, the clock (and gear selection indicator) light up and the bike starts easily.

While coasting to a stop - either in gear (with engine not firing) or with clutch pulled in), I have fiddled with ignition switch and the kill switch, but doing so has not brought the lights back on, rather only time (seconds) seem to bring things back to life.

This intermittent stalling is just that: intermittent. It may stall once every 20 times I come to a stop.

It appears to be electrical, but when it happens, I lose ALL power (again, even the clock). I have checked the basics like battery terminals and ground points.

Ideas?

Try to be specific rather than using terms like the basics.

Check the wiring harness plug to the Jetronic unit to be certain its pins and sockets are clean and tight, and that the unit's latch firmly connects the plug to the unit.

The ignition switch has a connector beneath the fuel tank. Inspect and clean it.

+1

It stalls on deceleration  at about 2000rpm but not at idle. 

Isn't that about when the injectors kick back in on deceleration? So maybe the jetronic. 

It's the hall sensors that tell the ECU what the rpm is,  so that might be two other places to look.

Check the exhaust valve clearances they probably are tight

daveyson wrote:+1

It stalls on deceleration  at about 2000rpm but not at idle. 

Isn't that about when the injectors kick back in on deceleration? So maybe the jetronic. 

It's the hall sensors that tell the ECU what the rpm is,  so that might be two other places to look

Maybe. How is the Jetronic unit tested? I had initially eliminated the hall sensor and Jetronic unit because when the bike stalls, I not only lose spark but everything which is electrical - headlight, clock, and alternating charging light - devices which are not associated with either the hall sensor order Jetronic unit..

RicK G wrote:Check the exhaust valve clearances they probably are tight

This was suggested at one point and clearances are spot on.

Laitch wrote:Try to be specific rather than using terms like the basics.

Check the wiring harness plug to the Jetronic unit to be certain its pins and sockets are clean and tight, and that the unit's latch firmly connects the plug to the unit.

The ignition switch has a connector beneath the fuel tank. Inspect and clean it.

Based on your comment I checked the Jetronic connector; pins are straight and shiny and the connector is properly seated. I will pull the tank tomorrow and check the ignition connector.

In between the hall sensor and the jetronic,  is the other computer, sometimes called the ignition control unit.

Clean the connectors anyway,  even if they look clean.

I checked the connector to the FI module, ensuring it was clean and securely fastened. While cleaning electrical connectors is always a good idea, I don't feel this is where the electrical fault is. I base this on the fact I can disconnect the FI module and the clock continues to function. Disconnecting the ignition switch connector or the ECU also has no effect on the clock (the clock is my canary in the coal mine).

Where the problem [may] lie is with a component that will kill the clock when disconnected. The battery terminals or frame grounds would be obvious candidates, but these connections are secure and rust and corrosion free.

Intermittent short between a battery terminal or terminal wire to the frame?

Earthing problem with the earth clamp under the fuel tank?

Can starter motor brushes and earthing issues bring this about?

On deceleration, at probably 2000rpm, the ECU, amongst other things, tells the fuel injection relay to supply power to the injectors, and other components,  straight from the battery. Consider cleaning the battery positive. It might look like one wire,  but there are three,  separate them and clean them. This large and sudden demand,  plus voltage drop at connections might just be too much. Since your brick stalls, on deceleration,  at 2000rpm, all those connections would do well to be cleaned. 

You mentioned bizarre readings, they might add to voltage drop as well.

Arrrr! I will have to remove the fuel tank and check. - yes, again! - the ground points (I just had the tank off a couple of days ago).

I’m having the same issue. Was the problem solved?

IanR, I don’t know if my problem is resolved. The last time the bike acted up it had been 4 months since the previous incident so while is seems to be resolved, tomorrow morning I may be back at ground zero. I would recommend reading the LONG post (search Electrical Gremlin) just to become aware of what I did, what seemed to work (for a while), and what didn’t work. My latest sense is to pay close attention to ground points. In short, I cannot point to any one thing that resolved my issue. Let me know what you have done this far.