K75C engine cut out

[franck]

For the first time in too many years to mention, I had to be recovered by a break down service!
The bike BMW K75C  (1985) 
The failure – the bike has been running beautifully all summer – recently 80km a day – with no hint of a problem.
About 100 yds from home, after about 70km journey, the bike just cut out – all ignition lights etc head lights all functioned.  After 5 minutes, I restarted the bike to get home.  I checked and cleaned connector to the ignition control unit.  Full of hope I started a second trip and this time a total cut out after 25 km – again no warning.  The bike turned over but would not start.
This morning I started the bike with no problem – I am not attempting another trip today!
Last week I dropped the bike (another strange history about that event) however that had no detrimental effect on performance.
It seems to me that the problem could be heat related, I don't think there is an engine cut out on over heating and the heat warning light didn’t come on. (the day was relatively cold 10°C and when I last checked the ventilator fan functioned)
I would welcome a few ideas before I start a search for the problem

[gazman]

Hall effect sensor?

[The Mighty Gryphon]

+1 on the Hall Effect Sensor.  You can test it by shooting some cold spray on it when the bike is hot and won't run.  You will have to remove the cover over them at the front of the engine yo do it.  I would remove all but two of the cover screws then go for a ride until it won't run. 

Alternatively you could heat them with a hair dryer while the bike is idling in the garage.  Then shoot the freeze spray when the bike quits.

THe Hall Effect Sensors(HES) are one of the few things on the bike that won't work when they get hot.

[Chaos]

what they said   http://www.ibmwr.org/ktech/intermittent-cutout.shtml  unfortunately it is a pricey little bugger, wasn't  there was a thread about fixing them on the cheap?

[The Mighty Gryphon]

Here's one:https://www.boonstraparts.com/en/part/bmw-k-75-rt-(k75rt)-hall-effect-sensor-1992&ID=B10000LRGE

Probably more to be found if you search eBay for "BMW K75 Hall Effect Sensor".

They can be repaired, but it involves drilling and riveting.  You may not want to take on such a fussy job.  I would go for a working used unit.

[franck]

I finally got round to testing the Hall effect sensor.  I remove the cover – 5 bolts finger tight and number 6 sheared off (well corroded) – needless to say the gasket fell apart; so far situation normal.   I let the bike warm up for about 10 minutes and then heated the sensor with an electric torch for about 10 minutes.  The engine didn’t cut out – the area around the sensor was very hot to the touch but I stopped for fear of melting wiring.  As I said on my original post both time the engine cut out there was no prior warning and the bike had been running very well at low and high revs.  I haven't given up the idea of the sensor as in my experience the solution to any problem will be the most difficut and/or expensive you can think of !   Any other ideas?  Are there any other sensors that can cut the engine?

[kennybobby]

Dirty contacts in the ignition switch could kill all power.

Cleaning k bike ignition switch

[franck]

The plot thickens - I had a talk to a local bike renovation specialist - his first question was had I replace the petrol pump.
The K75 had been sitting in a garage for many years, when I started work I cleaned out the tank and replaced the filter and hoses but not the pump.
Would you agree that a sudden cut out could be the pump?
Would it be worth changing anyway?
and going back to the Hall effect sensor - has anyone done the test with heating the sensor?  If so just how long should I keep on heating? - I  kept it up for a good 10 minutes and nothing happened - no coughs or splutters it just kept on ticking over.

[kennybobby]

...The K75 had been sitting in a garage for many years,

You left out that very important detail.
Yes the pump could be your problem.
The entire fuel system from tank to injectors must be checked, cleaned and verified to be clean and functional before you can rule it out.
The Hall sensors are okay, you heated them plenty.
Rule out the fuel system first, then air and compression, then electrical.

[franck]

Thanks one and all for the help.  After testing the Hall sensor it seems this is not the problem.   I have ordered a new pump - agreed a 30 year old pump could be the route of the problem.
Once last thought - the day before the cut out the bike was dropped with some loss of fuel.
Would it be possible that the breather (vent) pipe from the tank became blocked.  If this vent pipe the only way in which air can enter the tank could it be possible that  the lowering of the fuel level caused a partial vacuum hence cutting fuel supply?
May be I thought about the problem too much !

[kennybobby]

If a bike has been dropped, then it will be damaged.  You may not see it or think it, but it's there.

All systems will need to be checked from beginning to end, to verify structural integrity and operation.  For example, maybe the engine RUN/STOP switch on the right handlebar controls may have been jarred in a fall such that it is bouncing around shorting out internally during riding.  There are dozens of other such examples.

Stalling during operation on the roadway is very dangerous and could be fatal, be very careful until it is fixed and you have confidence it will perform.

[riots100]

It doesn't even have to be a bad pump.

On my bike, which had be sitting for many years,  silt had been accumulating for many years in the bottom of the tank.  So even though I replaced the fuel pump, filter and hoses, I too had an intermittent cut out problem (especially when the bike was warm - and the fuel level low)  The silt was getting stirred up and blocking the fine screen that surrounds the bottom of the fuel pump.  When the screen was blocked enough the engine cut out due to the reduced fuel flow.  When I then let the bike sit for a while, the silt would dislodge from the screen and settle back down to the bottom of the tank.

On these bikes, the fuel is constantly recirculated in the tank, and when the fuel level gets low there is a higher level of turbulence in the liquid in the tank, thus lifting what sediments that there might be and allow them to get drawn into that fine, fuel screen on the bottom of the fuel pump and impede the flow of fuel.

Rigorous cleaning of the fuel tank with solvents and vacuum cleaner completely eliminated any and all foreign contaminants and solved my problem.

Generally most solid state parts (like the HES) do not go bad if they have been working previously.  So I generally look for solutions in other areas before I will suspect the solid state parts on the bike.