Starts, Then Won't Start

[k]

When really cold, the 1985 K100 will start right up. Runs a bit rich, but it starts right up, and it more or less seems to run strong enough. I run it for a while, then turn it off. It won't start after that.

Ideas? Thank you.

[Chaos]

Bit sparse on info to diagnose.  Check out the repair diagnoses section.

[Laitch]

I run it for a while, then turn it off.

Does that mean you ride it or does that mean you idle it? How long or far is a while?

[k]

Does that mean you ride it or does that mean you idle it? How long or far is a while?

Some of each. 10 minutes.

When I first start it, it fires on first bump. When I shut it off then immediately try to restart it, I could crank it forever, and nothing happens.

[The Mighty Gryphon]

So, in other words, it won't start when the engine is warm.  First thing that comes to mind is that it is flooding due to a faulty engine temperature sensor or it's wiring.  How long before you can start it again.  A couple hours?  that would say the engine needs to completely cool down so it actually needs the richer mixture.

Pull the spark plugs when it won't start.  Are they wet with fuel?  Dry them off and put them back.  Unplug the tank connector and try to start the engine.  If it is flooded, it will fire and run very briefly after 10-15 seconds of cranking.  What it is doing is running on the remaining fuel after you have blown out the excess that was flooding the engine.

From there, you can move on to checking the temperature sensor.

[k]

The last time I rode the bike, I picked it up from a shop that changed the coolant sensor. It was parked until now.

[The Mighty Gryphon]

Interesting...  How did they know to replace the sensor?   Apparently, it was flooding from a too rich mixture.  Assuming they installed a good sensor, that means you may have a wiring issue.  The best way to check is to do a resistance check looking back from the Jetronic connector under the seat.

I can't tell you what pins to check,  but you should have about 2500 ohms when the engine is cold and around 300 ohms when at operating temperature.  A broken wire will make the Jetronic think the engine is super cold and needs an extra amount of fuel all the time.  I can't tell you what pins you need to measure, but there should be others here who can tell you, or you can do a search on temperature sensor testing to find other threads where these checks were run.

[k]

I told them to change it as preventative maintenance.

[The Mighty Gryphon]

In any event, it's the first thing that I think may be causing your problems. 

[Inge K.]

I can't tell you what pins to check,  but you should have about 2500 ohms when the engine is cold and around 300 ohms when at operating temperature.

http://www.motobrick.com/index.php/topic,10909.msg94705.html?PHPSESSID=b1389748b49312d5656e0d588aa440c5#msg94705

[dfx]

Pull the spark plugs when it won't start.  Are they wet with fuel?  Dry them off and put them back.  Unplug the tank connector and try to start the engine.  If it is flooded, it will fire and run very briefly after 10-15 seconds of cranking.  What it is doing is running on the remaining fuel after you have blown out the excess that was flooding the engine.

Hi all,
need to try unplugging the tank connector. Where is it again? Haven't touched my K100 for a couple of years and I've forgotten stuff I knew before.
Thanks

[Laitch]

Where is it again? Haven't touched my K100 for a couple of years and I've forgotten stuff I knew before.

Does this post jog your memory? 

[Martin]

The connector to the temperature sensor can become a bit dodgy/intermittent. It can become corroded, dirty and loose. Sometimes wriggling and applying pressure to the connection can overcome the problem temporarily.
Regards Martin.

[dfx]

Does this post jog your memory? 

Wow, that's scary. To think there are people out there who I've never met that know more about my past life than I do
I remember now, those were the days!

[Laitch]

Wow, that's scary. To think there are people out there who I've never met that know more about my past life than I do
I remember now, those were the days!

Speaking for myself, I know nothing about your battle skill during the Gallic Wars.

[dfx]

Well, I tried stopping the fuel pump to make sure the engine dried out. Although, first I took the fuse out instead of disconnecting the four pin plug on the left under the tank. That didn't work so I dismounted the fairing to get to the plug. That didn't work either. Then I took the plugs out and found I have no sparks.

So, to resume, bike goes off the road in summer 2018. Tried to start it in 2019 but wouldn't until I put fresh petrol in, then went with no worries. This summer, starts first time, a bit of fiddling to get the choke and throttle right, then it runs great and with no smoke even. Switch off after a few minutes, then it won't start again. 

[milq]

Maybe the Hall sensor is swelling with heat and causing an internal open circuit? I've seen coils work fine until hot, then do the same thing but I doubt you'd lose spark at all 4 cylinders with a coil doing it.

[k]

It was bad plugs, wires, and a fuel injector. A couple of things were out of adjustment. It runs great now.

[The Mighty Gryphon]

Wow! Almost 2 1/2 years since your last post on this thread.  That sure is some sort of perseverance.