K75 wont stay alive

[Rokah]

Hi! Long time no see.

after a few thousand of kilometers after restoring my bike, I've ran into the following issue

https://youtube.com/shorts/oBT3ZdBW1Ww?feature=share

As you see in the video, it cranks well, but then it will die with no reason, both hot and warm.

Both the fuel pump and the filter are pretty much new.

Do you guys know were to start debugging?

BR

Rodrigo

[Laitch]

Use your multimeter to find resistance values indicated in step #10 below and tell us what they are.

[frankenduck]

Disconnect and then firmly reconnect the L-Jetronic.

Possible suspects:

Fuel level sender connector.

Dirty ignition switch.
http://www.kbikeparts.com/classickbikes.com/ckb.tech/0.ckb.tech.files/ignition2vclean/ignition2vclean.htm

[Rokah]

Use your multimeter to find resistance values indicated in step #10 below and tell us what they are.

Hi! thanks for that! The result is coming at 1.488Kohm so basically around 45ºC maybe around 10ºC warmer than it should, but I dont think this is the issue

Disconnect and then firmly reconnect the L-Jetronic.

Possible suspects:

Fuel level sender connector.

Dirty ignition switch.
http://www.kbikeparts.com/classickbikes.com/ckb.tech/0.ckb.tech.files/ignition2vclean/ignition2vclean.htm

Cleaned the LJet tronic

But when cleaning the switch it looks like someone has already been here. Unfortunately the red screw was eaten by rust....see attachments

[The Mighty Gryphon]

I am pretty sure BMW puts that damn red Loctite on that screw in the ignition switch.  I know I broke the screw the first time I tried to take one apart to clean it.  Very careful heating with a heat gun or maybe a hair dryer can soften the Loctite enough to loosen it.  Take your time.

[Laitch]

. . . so basically around 45ºC . . .

If that is the engine temperature, the resistance is greater than on the mapped curve attached. Try it Thursday morning and see what it reads.

[Martin]

You can also carefully use a small soldering iron to heat up the small screw.
Regards Martin.

[Rokah]

If that is the engine temperature, the resistance is greater than on the mapped curve attached. Try it Thursday morning and see what it reads.

Correct, I did the calculation asumed linearly between 20 and 80º. But according to your curve 1.488kohm is around 37ºC which looks good because the bike was at ambient temperature in Spain yesterday.

[Rokah]

I am pretty sure BMW puts that damn red Loctite on that screw in the ignition switch.  I know I broke the screw the first time I tried to take one apart to clean it.  Very careful heating with a heat gun or maybe a hair dryer can soften the Loctite enough to loosen it.  Take your time.

You can also carefully use a small soldering iron to heat up the small screw.
Regards Martin.

Meh, it was rusted and it broke as soon as I put some force into it. It seems it has already been repaired before. I'm going to try to get another from a scarpyard and change the lock.

[Rokah]

I found a keyswitch in my local scrapyard, I replaced the lock and cleaned the mechanism. Ill try tomorrow and report back

[Rokah]

well, took it for a spin and it looks solved!

Will report back if it fails again,

Thanks Motobrick and specially Martin, Laitch,  The Mighty Gryphon and frankenduck!

[frankenduck]

These are the temp sensor values from the Bosch data sheet with a 5% tolerance:

[Rokah]

Still same problem! Takes a while to start when cold. No issues when warm. When cold it cut out on a long straight going down. The tachometer also went to zero although the engine was in gear

[Laitch]

Still same problem! Takes a while to start when cold. No issues when warm. When cold it cut out on a long straight going down. The tachometer also went to zero although the engine was in gear

Same problem as what? It isn't the same problem that started this thread, is it? The opening problem was it cranks well, but then it will die with no reason, both hot and warm. Now when it is warm, it runs well, correct?

Maybe the mixture is too lean to run well cold, but okay when warmed up. If it isn't the coolant temperature sensor interfering with the fuel mixture, test the crankcase to air box z-hose and the throttle body manifolds for leaks by spraying them with propane or carb cleaner while the engine is running. If you hear changes in the revs, you have found a leak.

Please be certain the fuel supply hoses connected to the fuel filter and the fuel delivery port are tight.

[Rokah]

Same problem as what? It isn't the same problem that started this thread, is it? The opening problem was it cranks well, but then it will die with no reason, both hot and warm. Now when it is warm, it runs well, correct?

Maybe the mixture is too lean to run well cold, but okay when warmed up. If it isn't the coolant temperature sensor interfering with the fuel mixture, test the crankcase to air box z-hose and the throttle body manifolds for leaks by spraying them with propane or carb cleaner while the engine is running. If you hear changes in the revs, you have found a leak.

Please be certain the fuel supply hoses connected to the fuel filter and the fuel delivery port are tight.

Hi Laitch! I've been trying to figure out a trend on when this happens.  For instance, last sunday I wanted to go out for a ride, the bike started and a few seconds after it died. Then it wouldn't start... I tried starting it for a few minutes with the occasional backfiring and when I was about to call it a day, it sprung to life. a few minutes after, on a long straight with closed throttle, the revs went to 0 on the tachometer and the bike died. As I was moving, a few seconds after it came to life again. Then I rode without issues for 120 miles, stopping for lunch and it worked well.

[The Mighty Gryphon]

Have you cleaned the ignition switch?  I had exactly the same problem with two bikes, and it was caused by the ignition switch and cleaning it corrected the problem.  That switch has been known to cause random engine starting and running problems, even shutting the engine off when running down the road.  It isn't the best design, and it doesn't take much dirt in it to cause problems.

http://www.eilenberger.net/K75S/IgnitionSwitch/

There is one caution.  The set screw that holds the switch together is bonded with that damn red Loctite.  You need a way to soften the Loctite to get the screw out.  Some have had luck with a fine point soldering iron to heat the screw.  You can also use a heat gun to heat the area around the screw but you need to be careful not melt the plastic.

[Laitch]

Have you cleaned the ignition switch?

In post #9, Rokah indicates the ignition switch was replaced and apparently cleaned; hopefully the contacts were polished and the mechanism lightly greased also.

When you have cranked the engine, Rokah, can you hear the fuel pump buzz within the tank for a couple of seconds after you stop cranking? If not, the four-pin fuel tank connector under the right side of the tank that supplies wiring to the fuel pump and fuel level sender should be inspected and cleaned with contact cleaner, or even replaced. Frankenduck suggested you check that in Reply #2 of this thread.

[The Mighty Gryphon]

Apologies.  I am having a harder time keeping track of what I have read and commented on.  I suspect the 60's are catching up with me. 

[Laitch]

The tachometer also went to zero although the engine was in gear

The tachometer is electronic. It records signals from one of the coils. When the engine stalls, there are no coil signals—no coil signals, no tachometer reading; going to zero is what happens.

[Rokah]

The tachometer is electronic. It records signals from one of the coils. When the engine stalls, there are no coil signals—no coil signals, no tachometer reading; going to zero is what happens.

But if the engine is turning due to being on the move with a gear engaged. I guess it should show something right? I'm thinking on some electrical issue related with ignition/fuel pressure

In post #9, Rokah indicates the ignition switch was replaced and apparently cleaned; hopefully the contacts were polished and the mechanism lightly greased also.

Correct

Apologies.  I am having a harder time keeping track of what I have read and commented on.  I suspect the 60's are catching up with me. 

No problem! I posted pictures of my refurbished contacts. It seems there are several of us having random cut-outs

[Laitch]

But if the engine is turning due to being on the move with a gear engaged. I guess it should show something right? I'm thinking on some electrical issue related with ignition/fuel pressure

Your guess is not accurate. One type of ignition problem with Bricks comes from the coils not firing for some reason. When the coils do not fire in a Brick engine system, the tachometer does not record revolutions. Zip, zero, nada. Take a look at this link.