Hi everyone!
   I have a new issue. I just completed a round of work on my 1985 K100RS. The work included rebuilding the ignition switch, replacing the starter relay, changing fuel and air filters, installing a headlight relay, and upgrading the ground wires.

  The new issue happened once during an engine run-up test in the parking lot yesterday and once during a ride this morning. After the engine had been at idle for a few minutes it wouldn't rev up. When I pulled the throttle, the engine stayed at about 1000RPM and sputtered and ran rough. It didn't die, though. Each time, shutting off and immediately restarting the engine completely cleared up the problem.

  There were many other times where the engine idled for a few minutes and then revved up fine. Other than those two instances the bike ran absolutely perfectly, over 20 minutes of idling and revving yesterday and a 40 minute ride this morning. Just those two times.

 So, I suspect a bad electrical connection. I'll pull the tank and check everything out tomorrow. Here are my questions:

• Am I correct in suspecting a bad electrical connection?
  
• Could it be something else? (If I'd messed up the fuel or air filter, I'd have a more consistent problem, right?)
  
• I'm thinking bad ground. Does that seem right?
  
• I'll check all my electrical connections, but are there any particularly suspect ones?
  
• Is there anything else I'm not thinking of?
  

   Thanks for your help, guys!
   -Jon

I wish that I has a definitive answer for you.
I had a similar problem, and after replacing the gas tank, fuel pump and filter (for another reason), the problem seems to have vanished.
The problem was:
When riding, at one point the bike was losing the engine power but was idling at the normal 800rpm.
If you tried to accelerate, it would die.
Shutting down the engine for a couple of minutes was clearing the problem for sometimes more than 1,000Kms.
Was it any water in the old tank trying to pass on the injectors? God knows!

First place I would go looking is the MAF sensor (air flow sensor) quite possible the flap is a bit sticky or had a tight spot that it has trouble getting past as it is opening.
Possibly the electrical connector for the MAF could be the problem but it sounds to me that the mixture ratio is being severly upset till it gets to the point where it wont run.

Update:
   I checked my ground connections. It did nothing. They were clean and tight. I made them cleaner and tighter. No help.

  Not only did it not help, the problem was MUCH worse today (a much hotter day). It happened pretty much every time I tried to accelerate from a stop. The only way I was able to limp home was by restarting the bike at every traffic light.

  Once I got home I pulled the tank to check the air flow sensor. I learned an important lesson: when you pull off the fuel line when the bike is hot, the fuel comes out somewhat more vigorously than usual.

 After washing the gasoline out of my eyes, I noticed that the air supply into the air box was only partially inserted. So the engine was drawing air from right on top of the engine instead of from in front of the radiator. Aha! I put the air hose back into place, put everything together and fired it up. Perfection! The bike ran completely perfectly.

  I went for a celebratory ride of about 25 miles, during which the bike ran absolutely perfectly... until I pulled back into my driveway, where exactly the same problem started happening again.   

  So I'm back to square one. Rick, in light of these new details do you still think it's the air flow sensor? Is there anything else I should check?

  Thanks as always for all your help, guys.
   -Jon

More update:
   I pulled the air flow sensor out this morning. As far as I can tell from an external inspection, everything seems fine. The flap couldn't be any smoother; it moves completely freely throughout its travel. I put the sensor near a vacuum to confirm the motion of the flap and it responded perfectly. So it isn't that.

   One suspicious thing: When I was trying to remove the hose from the outlet of the air sensor, the other end of the hose fell off the intake manifold box. I'm wondering if it might not have been completely seated there. If I partially pulled that hose off the intake manifold box while I was changing the air filter last week (right when the problem started) that would explain my symptoms, wouldn't it? The engine is getting some air through the sensor, but also some air through that improperly sealed joint which isn't being metered by the sensor. So, way too much air for the fuel level. Could that be what's going on?

   I'm also wondering if the sensor could be bad internally. I found a (purportedly) good one on eBay for $30, so I went ahead and ordered it. Maybe it will fix the problem, maybe I'll just end up with a spare. Do these sensors fail often?

  It feels like I'm closing in on fixing this one. Is there anything I'm not thinking of?

   Thanks!
   -Jon

I wouldn't go chasing any further as yet, just put the system back together with that hose well attached and see what goes.  My instinct tells me that all should be OK.

When I tighten that clamp I use a 7mm socket with a 1/4 drive ratchet handle which makes it easy to tighten. Make sure it is clean and dry as there is a very small lip to attach to.

The other end of the hose is almost impossible to remove with the air box in situ because the spigot of the AFM is also part of the inner attachment and the hose is fitted into the box like a grommet.

Another (final?) update:
   I checked that all resistance measurements on my MAF sensor were as they should be. The air flow readings were perfect. Air temperature was reading a little high, but only by about 5 degrees. Should be fine, right?

   So I put everything back together all clean and tight and perfect, as Rick suggested. Fired the bike up and immediately had the same problem. It idled almost okay, but coughed and sputtered and almost died as soon as I touched the throttle.

   I spent about ten seconds lost in deep, dark despair and then I had a thought: the problem definitely seems like the FICU is not getting a correct signal from the MAF sensor. I confirmed the sensor is fine. The air hoses are all tightly sealed, the electrical connection at the sensor is clean and tight. What about the connection at the FICU? I gave the plug a little shove and fired up the bike again. Perfect .

   So this morning I pulled the plug at the FICU and gave all the connectors a good cleaning. I took the bike out for about 25 minutes and hit it with every kind of use I could think of: stop and go traffic, wide open high revs, idle for 3 minutes followed by high revving, etc. The bike was flawless.  

   I'm still a little suspicious since the problem has been intermittent at times, but I think I got it. I'm planning on leaving work early tomorrow to take the bike out for an hour or two to make sure. I'll let you know how it does.

   Thanks as always for all your help. You guys are awesome!
   -Jon

My experience with FICU on a couple of Ks......

The plug can get dodgy and produce a dodgy connection. I solved this on a K75 last year simply by holding the plug in. My own 84 K100RT did this earlier this year and I found the plastic part on the FICU that engages the plug had a hard to see crack. Swapped the FICU unit from the 92 K100LT and all was solved.

Definitely final update:
   I rode over a thousand miles in two days this weekend, with no hint of trouble. The poor connection at the FICU was the issue. Cleaning the connection cleared the problem 100%.

   And, by the way, the bike is SO MUCH FUN! I'll post some photos of my trip in the general discussion forum.
  -Jon

That's really good news Jon. They are so good to ride when going well and fortunately that is most of the time.

Excellent news!

Glad you are having fun now.

Good to hear that it was simple fix with a lot of headache to find.
Last week, over the Internet I troubleshooted a problem for somebody owning a K75 in British Columbia.
By asking the right questions and getting the right answer, I was able to pinpoint the problem to the ICU. This is a very rare occurence on a K bike, but after the owner bought a used one on Ebay, the bike fired right away.
Detailing the problem when you post and honestly answering what could seem as a stupid question always help to pinpoint the root of a problem.
You did it and it worked for you.
Enjoy the ride.