My K1100LT has run perfectly fine all year. However, during the fall, it started to become slightly less eager to start, though it wasn’t really an issue. That is, until it suddenly wouldn’t start anymore. When I turn the ignition on, I can hear the fuel pump running, and I hear the return fuel trickling back into the tank. The starter motor turns over, and sometimes it seems like it’s about to catch, but in the end, it doesn’t start.
It’s worth mentioning that the motorcycle is stored outside.

Checked:

• Electrical connections (all look good). I disconnected and reconnected the injector plugs (haven’t measured resistance yet), and the spark plugs look fine.
  
• All spark plugs are sparking (I can’t check the timing).
  
• Water in the tank? No unusual signs around the fuel cap. I disconnected the fuel line at the injector rail, and there was no water in the fuel. When I turn the ignition on (without starting), about 50 ml of fuel comes out of the line each time with a strong stream. I haven’t tested how much fuel comes out during starting.
  

After reconnecting the fuel line to the rail, I tried starting it again, and suddenly, it ran! I let it idle for about a minute, turned it off, and tried again, but nothing—absolutely nothing.
I disconnected and reconnected the fuel line again, but it still won’t start...

What now? The ignition seems fine, the fuel pump appears to be working, but something seems wrong with the fuel delivery.


Has anyone experienced something like this before?

I have already found this great guide, which I will go through in detail: https://www.kforum-tech.com/forum-area/__Files/electrical/EFI/bike-wont-start-EN.htm  . Based on this I am suspecting the various sensors that affect the Fuel Injection system. So next task will be to disconnect and reconnect all elements of the FI system.

I'd suspect the temperature sensor behind the radiator, if only for the reason that I've had two go faulty over the years with pretty much those same symptoms.

ewout wrote:I have already found this great guide which I will go through in detail  Based on this I am suspecting the various sensors that affect the Fuel Injection system. So next task will be to disconnect and reconnect all elements of the FI system.

It is a great guide; however, your K1100 has a different engine control system. The guide you have referenced is dedicated to K100 and K75 2-valve models using an LE-Jetronic engine management system. Your Brick likely uses a Motronic 2.1 system.

One significant difference between the systems is your Brick likely has the ignition output stage illustrated below if your Brick was made in or before 07/93. If it has an ignition output stage, clean it because you store your Brick outside and the ignition output stage is sensitive to oxidation. It is located on the battery carriage. Its major components are plate #8, part #7 and the plate on its reverse side, and the connection assembly parts #1 and #2. The interfacing surfaces of plates #8 and #7 should be lightly dressed with fine emery cloth, cleaned with electronic cleaner, then given a light coating of heat sink paste. The output plug connector should be cleaned with electronic contact cleaner also.

If that doesn't clear the problem, there are other systems to explore such as the coolant temperature sensor Dai has recommended. 

I agree that the pesky temp sensor at the "gooseneck" can cause irregular starting and is the most likely candidate for a thorough cleaning (the tip gathers corrosion) or replacement. Another thing to consider is the fuel filter being clogged. It is a maintenance item, afterall. After checking and cleaning the temp sensor - the first best guess - remove the easy to access fuel filter and see if it can easily be blown through. Slow to start, a gradual issue...

Flaky side stand switch?

I had that on my 93 LT. Fixed it by putting a short section off vac hose on the tang that pushes on the switch. Easy peasy.

Thanks for all your valuable responses!

I was all set to look into the temperature sensor, which I also suspected based on other posts and the guide. But one of the first suggestions in the guide was to disconnect the idle switch and try starting... The switch is very easily accessible and after disconnecting it, the engine started immediately!

So after taking the switch off and cleaning it a bit, used some contact spray on the turning part, I put it back and now the engine starts like it used to.

Result!