85 K100 intermittently starts and dies quickly

[McQuinn]

It's a 1985 BMW K100 that's been converted into a cafe racer, no gauges or warning lights.  Sometimes it'll start and run fine, I took it for a 20 minute ride 2 days ago and it ran perfectly.  Got back and parked it, and went out every 30 min or so to start it and it was fine.  Got home from work yesterday and it would start for a second and die.  Got home from work today and it fired right up and ran great then after a few min it died and would do the start and then die thing.  I sprayed starting fluid in the intake and it fired up and ran fine and is starting and running now.

What I've done so far:

Tested cylinder compression and every cylinder was around 152.
Fuel pressure is between 33-36psi.
New spark plugs and fuel and air filter.
Had injectors ultrasonically cleaned 6 months ago.
New crank case vent hose and checked all vacuum hoses for cracks.
Cleaned every connection I could find and made sure they were tight.

When it does the start and die thing if you use starting fluid it'll start and run for 10-15 seconds and then die so it's something fuel supply related, I'm wondering about the ECU or hall sensors.  Just looking for any advice if someone has had the same symptoms before.  Thanks for any help, let me know if any more info would be helpful

[Laitch]

You wrote in August 2019 that it was running great. Did its decline into starting failure happen slowly or suddenly?

[McQuinn]

The first time it happened was probably 4 months ago.  I rode it to work with no issues and then when I tried to start it to go home it would start and die.  After 30 min or so of checking stuff and trying to start it it finally fired up and I got home with no issues, and it's been intermittent since then. Shortly after the first time it happened I took the tank in to have a dent fixed and repainted and while that was being done I checked all the connections and changed the fuel filter and spark plugs. I haven't ridden much because I don't want to get stranded so it's been limited to starting and running in the garage or taking short rides close to home.  I was busy at work and getting my son off to college so it's been on the back burner for the last few months.

[Laitch]

What's the resting voltage of the battery?

[Martin]

Check the fuel lines for crimping. Fuel might be getting through enough to fill the rail allowing it to run until the fuel supply in the rail is exhausted.
Regards Martin.

[daveson]

Fore pin?

Wiggling it might start or stop the problem.

[McQuinn]

Battery voltage while motor is off is 11.77, while idling 11.37, don't have a tach but what I would assume to be 4000 rpm the voltage is 13.15

Fuel lines seem good, when it does run it runs great.  I feel like right now, while it's starting and running I could go for a 15 mile ride and it'd be perfect, but then tomorrow it'll revert back to start and die. And then the next day it'll run fine.  Makes me think bad connection somewhere but there's no rhyme or reason to it.

I've checked and cleaned the fore pin (that's the connection under the gas tank?) , it seems like a nice, tight fit.  I'll fiddle with it next time it does the start/die thing and see if that helps.  I've also fiddled with the start button switch but that doesn't seem to be the issue

[daveson]

Sounds like you need to charge the battery and replace the alternator regulator.

[Laitch]

Battery voltage while motor is off is 11.77, while idling 11.37

Your battery is too weak for the task, as daveson indicates. That can't be ignored. Test voltage regulator output, too.

[McQuinn]

Thank you, that's not something I had not considered, I was focusing on fuel supply but it's all a tangle web.  I'll charge the battery overnight and test the voltage regulator in the morning

[McQuinn]

Thank you, that's not something I had not considered, I was focusing on fuel supply but it's all a tangle web.  I'll charge the battery overnight and test the voltage regulator in the morning

Holy double negatives, I bungled the hell out of that 

[stokester]

Your battery is too weak for the task, as daveson indicates. That can't be ignored. Test voltage regulator output, too.

It is a common misconception that a battery reading near 12 volts at rest is fully charged when in fact it is less than 50% charged.

A 12 volt battery consists of six cells of 2.1 volts so a fully charged battery should be 12.6 volts.  While a multimeter will give an indication of the battery's health the only way to really determine the state of charge is doing a load test because a meter reading after a charge or ride will have a surface charge giving a higher reading.  A minute or so with engine off and headlamp on will normally remove this charge.

[rbm]

Since this is a cafe conversion, should we assume that you've replaced the charge indicator with a LED?  If so, the engine RPMs need to be above 3000 to start the charge.  That's why you see low voltage at idle or off-idle, yet 13V or so at 4000 RPM.  The charge indicator really should be a standard incandescent bulb; there's no reason to use a LED.

[Laitch]

Since this is a cafe conversion, should we assume that you've replaced the charge indicator with a LED?

McQuinn has indicated the moto has no gauges or warning lights. and you've made a good observation regarding alternator activation, rbm. The moto was running well last August, perhaps activating the alternator with residual voltage occasionally but not consistently. In that scenario, over the year since then, the battery would probably have been slowly discharging and now finally has given up the ghost.

[McQuinn]

Ok so fully charged battery and it's back to start/die.  It'll idle roughly for a few seconds if I keep holding the start button and will die immediately if I apply any throttle.  If I spray starting fluid in the intake it'll idle and run normally and rev up if throttle is applied and then die shortly after.  I swear this thing is on a day on/day off schedule, it'll start and run fine tomorrow and show no symptoms

[Laitch]

Ok so fully charged battery and it's back to start/die.  It'll idle roughly for a few seconds if I keep holding the start button and will die immediately if I apply any throttle.  If I spray starting fluid in the intake it'll idle and run normally and rev up if throttle is applied and then die shortly after.  I swear this thing is on a day on/day off schedule, it'll start and run fine tomorrow and show no symptoms

Nothing different is happening. Perhaps the battery is unable to hold a charge, as was indicated early here. Test the voltage right after it stalls.

Without the bulb that rbm describes in the charging system, the alternator might not activate then the engine stalls. The battery alone cannot run the load because it is discharged to some degree. Sometimes the alternator does activate on residual voltage and the engine will run. The battery will be degraded by intermittent discharging from faulty alternator performance and eventually won't hold a charge at all. I think your engine is operating by chance right now from these conditions.

Your job is to determine whether the condition described is at the root or some other condition.

[McQuinn]

Ran a test using starter fluid so it'll idle for a bit.  Battery was reading 12.4 and dropped to 11.3 while starting, idled at 12.1-12.2 until it died and then returned to 12.4 when I removed the key. Voltage jumped up to 13.1 when I revved it.  Did this a few times trying to see if there was some drop when it stalls but the lowest reading was 12.1 and then a return to 12.4 after removing key.  I did order a voltage regulator because I don't think the alternator is charging properly at idle, I'm not sure it's the issue at hand but the alternator doesn't seem to be charging properly and it's an inexpensive part to replace.

[rbm]

Can you confirm or deny that you have a charge indicator on your bike?  Or that you have installed a resistor between a power lead and the blue wire on the alternator?  This is important as both I and Laitch have mentioned.

[McQuinn]

There is no charge indicator or a resistor that I'm aware of, I'll check the power lead to see if a resistor is installed

[rbm]

Ok.  So, if you don't supply power to the alternator stator coils during operation, then reliable charging will not occur.  It won't matter whether or not your regulator is changed out.  The configuration you have now - blue wire dangling in space - is exacerbating your poor running condition because your only source of power, the battery, is presently incapable of supplying the current needed to sustain the motor's operation. 

Rather than order a new regulator, purchase a 3W lightbulb or 150 Ohm 3W resistor.  Connect whatever you've chosen in series between a source of power on the bike and the blue exciter wire on the alternator.  This will cause the alternator to function properly and maybe your problems will be rectified. I personally would recommend a light bulb so that you have an indication of the charge state of your battery.

[The Mighty Gryphon]

Ran a test using starter fluid so it'll idle for a bit.  Battery was reading 12.4 and dropped to 11.3 while starting, idled at 12.1-12.2 until it died and then returned to 12.4 when I removed the key. Voltage jumped up to 13.1 when I revved it.  Did this a few times trying to see if there was some drop when it stalls but the lowest reading was 12.1 and then a return to 12.4 after removing key.  I did order a voltage regulator because I don't think the alternator is charging properly at idle, I'm not sure it's the issue at hand but the alternator doesn't seem to be charging properly and it's an inexpensive part to replace.

12.4 is nowhere near a full charge.  When I was working on boats, we didn't consider anything below 12.8v to be a full charge after knocking off the surface charge.  Put the battery on a good charger.  What is the voltage at the battery terminals with the headlight on and the engine not running?  That is a redneck load test.  You should have at least 12.5 to 12.6v if the battery is good.

If that battery has been sitting for any length of time with a low charge, the plates are probably damaged and it is now unable to take and hold a full charge. 

A good alternator should be able to go as high as 13.8v into a fully charged battery when the engine is running above 2500rpm.

As RBM mentioned, without an incandescent charging lamp, you need to rev to at least 3000rpm, maybe higher, to self excite the field windings and get the alternator to start charging the battery.

13.1 volts when you rev the engine tells me that the battery is loading down the alternator because it has a low charge, or there is something else on the bike drawing a lot of current.  It could also mean that the brushes in the alternator are worn and not making good contact.

[McQuinn]

Good info, there is no resistor so I'll take care of that issue first and report back.  The battery is roughly 1.5 years old, I trickle charged it overnight but I'll fully disconnect it and fully charge it.  Thanks for the responses, very helpful

[Laitch]

The battery is roughly 1.5 years old,

That age isn't too young for the battery to have been damaged by mishandling. How many times it has been severely discharged then needed to be recharged and the timeliness of recharge is relevant.

[Martin]

After it has died pull the spark plugs are they wet or dry?
Regards Martin.

[McQuinn]

The battery has never been run down and recharged, this is the first time I've put a charger on it

I ordered the resistor, couldn't find one nearby.

Have to change the oil pan gasket in the wife's car tomorrow so I had to move the bike.  Figured I'd try to start it and see what happens and of course it fires up and runs great after not starting all morning.  Damn bike has a mind of it's own, just does whatever it feels like.  I moved the bike and disconnected the battery and put the charger on it, I'll check in the morning to see what it reads.  Thanks again for all the suggestions