Alright, boys, here we go. 1991 k100rs 16v.

I rode about 15min to work, all was normal, smooth, and good.  Rode 15min home from work, and about half way through I noticed the bike missed a little when I gave it some throttle. So like anybody curious would do, I gave it throttle again, and whaddya know, it missed/stumbled again.  It got worse as the ride went on.  It idled ok at the stoplight after I got off the highway, but as soon as I'd rev it at all it would stumble and try to die.  I had to ease in to it *really* easy to make it home.  Constantly almost dying in low rpm at that point, missing/stumbling alot.  Got home, turned bike off, bike won't start. That was a couple days ago.

I've checked: fuel / air / spark / compression.  Air was good, spark was normal, all 4 injectors were equal in spray and seemed normal.  -Possibly- it was too much fuel?, but I don't think so in comparison to youtube videos of people's injectors spraying.  Compression is as follows, cylinder 1-4 in psi: 120, 110, 125, 120.

What else could it be?  As I'm typing this, I'm leaning toward the Hall sensor, but that's usually heat related and goes away when the bike cools off.  Could it be a coolant temp sensor malfunctioning?  Could the computer have bit the dust?

It cranks over normally, nice and strong, it just doesn't catch and start..  It never sounds like it "almost" starts either, just cranks over.

Thanks in advance.

loose connection at the ecu ????

unplug it and refit it ?

The injectors are triggered by the pulse from the 2-3 hall sensor so it is working and if the 1-4 sensor was cactus I would expect it to try to fire. Disconnect the fuel tank connector to stop fuel delivery and crank it over with open throttle to dry the plugs off then reconnect the pump and see if the plugs get wet. this can indicate the temp sensor is failing or failed.

Those compression readings are a bit low for a 16V engine but it shouldn't prevent it running or starting.
Have you checked to see if there is a big air leak as that would cause a stumble when opening the throttle and if it got worse quickly it mat indicate a rapid deterioration of the intake system in some way.

I checked the ecu, connection looks good, nice and clean, refit and no change.

I haven't checked for a huge air leak, but i don't imagine that would stop it from starting or even trying to catch.  I can take the airbox out of the equation and check the rubber connection below the butterflies.

The plugs were wet when I pulled them out after cranking it.

temp sensor ?

Sidestand down?

nope to sidestand. 

@charlie99, is there a way I can test that without taking it out and boiling it or something?

perhaps you could probe the connections going into the computer with a mutli meter  set on 20,000 ohm scale

not as familiar with k16 valve models to tell you which pins to measure (as I am with k 8 valve models )  but if it reads above 5 000 ohms there is something definatly wrong with either the temp sensor or the cabling going to it ... we have seen corroded terminals on the temp sensor before ...needing a clean up ....and is a fickle thing .. good luck

charlie99 wrote: not as familiar with k16 valve models to tell you which pins to measure

The pinout you find in the following link:
https://www.k100-forum.com/t5169-cooling-fan-stopped-running-getting-hot-now#56956

The water temp sensor is connected to #21 & 32.

good stuff yet again inge

cheers !!

What happens when you just roll on the throttle?

When was the fuel filter last changed?

Also check the connector on the Control Unit (Ignition Amplifier) located on the front of the battery box.

This has given me a few issues on my k1

When i roll on the throttle while cranking, it just cranks with the throttle open, no change.

Fuel filter was changed a year or so ago.  I confirmed there's a good fuel spray and fuel going back in to the tank.  I had the thing bust open a fuel line inside the tank when the last filter was clogged, and it ran to the very end, so I know it's not that!

I'll check the rest of these things when I get home tonight and post.  Thanks, you guys rock!

Update: soooo, i didn't -really- do anything.... but it just started, and idled normally. (side note, it's about 55 deg F outside at night, if this makes a difference in some sensor readings to the ecu.)

I fiddled around with the ignition amplifier, but didn't actually fully unplug and re-plug it in.  And then I unplugged the gas tank and started it.  It started and idled for a couple seconds and then died due to no gas.  I started it a couple more times and it would catch for a second then die.  I did this until it would only crank and not catch, clearing out what I assume would be extra fuel.  The fact that I was able to do all this with the tank unplugged, was it flooded?  I wonder if the temp sensor went bad and put it in to the "start" mixture while i was riding before and caused it to die?

Anyways, it's starting and idling normally for now, I'll be able to ride it around a bit tomorrow.  I'm not convinced it's fixed completely and/or permanently...  Maybe I could clean some contacts/connections? Even then, I'm not convinced!  We'll see..

Sounds like a bad temperature sensor connection. While I only have a 2V I've been caught out twice with a bad connector. I had to remove the fuel pump fuse to start it  and ram the fuse back in before it died. A mate who was a BMW tech told me when I got the Brick to clean all the connections. This connection was the only one I didn't do due to it being hard to access. Learned my lesson after the second time and cleaned it properly. This has been the only electrical connector fault in twenty years.
Regards Martin.

It sure is starting to sound like a temp sensor or connection.

Connector, rather then the sensor itself?  I know the sensor that's in there is only a few years old as I had issues with it awhile back and had to buy a new one.

So does that mean that the connection is just janky, and the computer loses the signal from the connector and puts itself in to.... start mode? Some kind of different mixture?

The sensor we're talking about, to confirm, is the one that's right behind the radiator on that little metal post that coolant goes through?  So I've gotta get in there, unplug it, clean it up, plug it back in, and then see what happens?

Thanks much

With mine it was just a matter of cleaning it with something like Deoxit. However against general consensus after I clean my connectors I put a smear of heavy silicone or dielectric grease around the connectors. I do this in the belief that it will keep out air and moisture which eventually leads to corrosion and bad conductivity. The other point of view states that by using grease you can induce bad conductivity by insulating the connections. Whenever I pull apart any connector I try to give it a clean and grease. As I have stated previously it has seemed to work with only having two connector problems, which occurred on connectors not treated. However your call on whether to grease or not to grease, that is the question.
Regards Martin.

*update* bike rode normally for a couple weeks after i had got it working again. A couple nights ago I was on the free way and the same symptoms suddenly returned, this time with a fury.  I could see the flash from the backfires on the ground in front of me.  I pulled off and ended up having to load it in to a van after a gig (musician).

I'm 100% sure it's the ignition amplifier at this point.  I tried a few things to no avail, and as soon as I unplugged the ignition amplifier and plugged it back in, the bike started right up.  Same as last time, except last time I didn't completely remove and clean the connection.  Whether or not it's the part or the connection, I ordered a new one with heat sink paste as well, cause I'm sure it can't hurt!

Ride a bike to a gig?  You obviously aren't a drummer, or maybe you have your own drum tech...

aaaaannnndd we're back. (16valve k100) I'm posting here but I'm not entirely sure it's the same issue.  The symptoms are very similar, missing under load, idle is a little off, etc.  The bike ran great for a few weeks, since the last post in here, but alas..

Here's what I did tonight:
-tried known-good hall sensor, no change.
-tried different coils, no change.
-compression test, same as before, cylinder 2 is lower by about 10-12 psi, but always has been, the rest are ~120psi
-cleaned / gapped plugs, no change.
-tried different control unit, no change.
-checked injectors via squirting them on to cardboard, all look same.
-re-seated computer connection, no change.
-opened airbox to bypass air filter, no change.
-checked sparks, all 4 plugs spark normally.
-cleaned up / re-seated water temp sensor connection again, no change.

Regardless of all this, here is what I ultimately ended up finding: at idle, it would do this chuff / small backfire type thing moderately consistently.  By unplugging the injector connection on each cylinder one by one, I believe I found the culprit.

Each injector I unplugged (one at a time) would cause the idle to drop as expected, EXCEPT cylinder 2 (the one with lower compression).  When I unplugged this one, the chuff / small backfire would stop happening.  Plug it back in, chuff --- chuff --- chuff.  Unplug it, no chuff.  This was consistent and repeatable.  With this information, I come to the conclusion that cylinder 2 is the one missing when under load.

With all the things I checked already, I don't know what else it could be, except maybe a valve adjustment.  I would think this would have come on gradually though, rather than suddenly.  I don't hear any clicking or rattling out of the ordinary.  The bike has 192,000 miles on it, and I think my last valve adjustment was probably 80-90k miles ago...  might it be time?..  If that doesn't do it, what could possibly be causing this?

Could I have just gotten a crappy control unit?  I bought one of those Enduralast ones.

Do a injector swap and see if the problem moves.
Regards Martin.

My first thought when I read this was valve adjustment has closed up a touch and with the #2 compression being a tad low that is where I would start looking. As Martin has said the injector could be the problem. It could be that the port has been getting a carbon build up with the inlet valve riding a bit and that would cause some blow back hence the chuff.

Not a big job to swap say no 2 and 3 injectors and see if the problem follows......

Update: runnin' smooth as a hot knife through butter!  As I was riding around earlier, I tried to pay close attention to the miss and see if there was a pattern.  Well, the pattern was that only certain ranges on the throttle would cause it to miss, and consistently.  This meant the mixture was off in a cylinder for those throttle ranges, which in turn lead me to look at the injector(s) again per Martin's recommendation.

Rather than swap, I just pulled 'em all and cleaned them, and checked resistances.  All checked out, cleaned up nice, and it's better than ever.  Thanks all!


Probably due for a valve adjustment either way..

  Always good to see a positive result.
Regards Martin.

Rabidchiwawa007 wrote:Update: runnin' smooth as a hot knife through butter!  As I was riding around earlier, I tried to pay close attention to the miss and see if there was a pattern.  Well, the pattern was that only certain ranges on the throttle would cause it to miss, and consistently.  This meant the mixture was off in a cylinder for those throttle ranges, which in turn lead me to look at the injector(s) again per Martin's recommendation.

Rather than swap, I just pulled 'em all and cleaned them, and checked resistances.  All checked out, cleaned up nice, and it's better than ever.  Thanks all!

Good to hear you got it sorted.  I run a bottle of Techron Fuel System Cleaner with a tank of no corn gas once or twice a year to keep things working nice.  On three bikes over a total of 60,000 miles I haven't had to pull the injectors yet.