BMW K 75 Problem: mysterioes loss of power

I recently have a strange problem with my girlfriends BMW K75 from
1985 (yup, you got it right, she owns the bike, I have the problem ;-)

The bike starts fine, usaually runs well but sometimes it seems to
experience a sudden loss of power while driving. I then still can
shift gears in the normal way but no matter which gear I use, no
matter which speed I'm at, the engine won't go over 5000 rpm (or so,
at least not even close to the "red area")!

This behaviour doesn't seem to follow any pattern: rain, heat, warm or
cold engine,lights on/off ... all that doesn't seem to have an impact.

Once I stop the enigne(sometimes for a minute, sometimes for hours)
the engine works fine again until ...hups, there she goes again.

A friend told me, this problem could be connected to the "failure
memory" which, once it gets a signal about e.g. a broken sensor,
limits the maximum power of the engine. This way you are able to reach
the next garage, though with limited power only.

Does this problem mean anything to anybody? Experiences?

Thanks for any input!

 

(Markus Alt) wrote in

It could be a plugged vent in the fuel tank. When it does this, try
opening the fill cap; if there's a sudden inrush of air and the
engine then runs fine, your tank vent may be plugged and you're
getting a vacuum in the tank.

-Keith

 

Jeez..... when ignoramuses offer help ;-))

 

Probably useless information but a friend of mine had somewhat
similar problems. His got so bad that it would not idle or start.

In his case the cause was one injector that was bad and the
pressure control valve (a kickback that opens back to the tank)
was faulty and dumped the injection pressure.

 

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(Markus Alt) wrote in message

There's a throttle switch which (apparently) sometimes becomes jammed-
this causes the injection unit to 'believe' that the throttle is shut.

Not sure if this is it and it's not a fault I've had. Just one I've
read about!

 

Don't forget the fuel filter, particularly if the bike sat for a
considerable length of time at some point (since the filter was last
changed).

Mark Hickey

 

Well, that's what I meant. every single K bike ever built has been fuel
injected, and they've been making them for over 20 years.

Similar.....

 

That's thinking outside the boxer!

But near as I can tell, a stuck switch would prevent the engine from
exceeding 2000 rpm.

 

Taking into account, how many cables and stuff I checked on this bike,
no idea/ information is useless for me So I will definately check
this next weekend, thanks!

 

Hmmmmm, looks important. Can you detail this for me?

 

That's thinking outside the boxer!

Hmmm...it goes up higher in my case. But still - thanks - another
thing to check on my list.

I wonder if anybody has heared about this "failure memory", where all
problems get recorded (modern cars have that too, so that your garage
can hook it up to a laptop and see, where the problem(s) come from).
Apparently the K75 has such a thing. Question is: where, can you get
software/ cables to read the memory yourself and has anyone experience
with this kind of stuff?

If not, I will test all the hints given here.

 

I don't think the L-Jetronic logs faults. BMW has a hand held
diagnostic tool, but I think its just an intelligent multimeter.

Start off by checking the plugs. If one plug doesn't match the
others, you could isolate it down to either a single coil or
injector problem. If there's no backfire during power loss,
I would lean toward the injector. I've had the shutdown
solenoid on a Roosa Master injection pump go bad and the
behavior was similar. (The solenoid would fail only when hot.)
The injectors are "ganged", so a controller problem would affect
all three cylinders.

 

All right, plenty of things to check - thanks folks! As soon as I
found the time to do so, I will let u know about the results.