Hey there!

My K100 RS- 85 is having trouble at low rpm and low power load.

The idle is somewhat uneven, but worse, the engine suddenly misses out, its hacking for a fraction of a second, looses momentum and then quickly returns to pace. As if you would chop the idle with an axe, or cutting it with scissors.

This can happen fx every five or ten second, or other times only every minute or so. At speeds above ca 50 kmh and at high power loads the engine runs completely perfect.

I've done fuel tank restauration, new fuel hoses, new ignition cables, new plugs.

Has anyone experienced similar issues? Could a faulty Hall- transmitter cause this problems? Or dirty injektion nozzles?

Grateful for any tips!

Brg/ Örjan.

Have you directed a narrow spray of volatile carburetor cleaner, propane, Mapp gas, or acetylene around the z-hose connection from the crankcase to the plenum, each throttle body's air intake hose from the plenum, and each throttle body's intake manifold connection at the cylinder head to determine if doing that raises the idle? There could be deterioration causing air leaks at one or more of those points fouling the air:fuel mixture.

Sounds symptomatic of cracks/leaks in the boots between the throttle bodies and the cylinder head.

They will leak at idle and then when the vacuum increases at higher RPMs the vacuum kind of seals them.

Part 11611460408

As Laitch suggests spraying some unlit propane from a torch around them at idle is a good way to diagnose.

Hi, thanx for reply, ill try the gas- test, however, since the engine running at idle is kind of "chopped up" very briefly, does that really point at air/ fuel problems, or rather ignition issues? Ive read in my Clymer that the engine indeed works with different ignition programs at different rpms, hence my question.
Thanx!

Check Z pipe, check if you have a vacuum pipe from no 1 throttle body, early 85 Ks have one, replace it with a vacuum cap.

Balance throttle bodies too, but also check the throttle position sensor [TPS]. My 84 RT did what yours is doing and that cured it.

Early Ks suffer coil and HT lead failure too. BSK Speedworks sell a replacement coil and HT lead package that is very good value.

Z- pipe is ok. What is "HT lead" ?

American English it is "Spark plug wires".

slowandlow wrote:. , , does that really point at air/ fuel problems, or rather ignition issues?

Maybe you should just go where your heart leads you.

The air/fuel ratio is most sensitive to leaks at small throttle openings like occur at idle and in low speed/light load situations .  These leaks can lean out the mixture to where it won't reliably ignite in the cylinder.  As the throttle opening is increased at higher rpm/loads, this small amount of unmeasured air that is leaking affects the overall mixture much less so engine operation improves. 

Also, at heavier loads and higher rpm with the throttle wider, the manifold vacuum is less so there is less suction to pull unmeasured air through the leaks into the intake manifold.

Aha, ignition cables? Have replaced them with a german made package, still got the problems. I will check the coils and see if i can measure them to get wiser.

battery low

Nope, new battery: 13,4 Volt at idle, 13,8 volt at higher rpm. How does the Hall- transmitter work? Can it be mechanically worn out, or does it either work or doesn't work at all?

slowandlow wrote:How does the Hall- transmitter work? Can it be mechanically worn out, or does it either work or doesn't work at all?

Your Brick has two Hall Effect Sensors. If one of them failed, you would be unlikely to characterize the performance of the engine as "runs completely perfect" no matter the speed. "Completely perfect" would also be an elusive condition with a bad coil.

slowandlow wrote:Nope, new battery: 13,4 Volt at idle, 13,8 volt at higher rpm. How does the Hall- transmitter work? Can it be mechanically worn out, or does it either work or doesn't work at all?

A Hall Effect device outputs a square wave. It switches ON or OFF, according to whether a magnetic field is sensed.
It is more likely to work properly or fail completely, rather than partially work.

Where is the other Hall sensor? Ive also been told it could be the throttle position switch (next to the no 4 intake) since a bad switch could influence the engine performance. And also, a bad connector down at the coolant temperature sensor cold disturb engine/ ignition, since this sensor- when connector completely removed/ pulled out- will shut down the engine as an overheating protection action. Anyone having opinions to that?

slowandlow wrote:Ive also been told . . .
Anyone having opinions to that?

I'm glad you asked!

Orderly system testing beats random theorizing any day. Could a faulty coolant temperature sensor be involved in this? Yes. So could intake air leaks, karmic debt or ancestral curses.

Hall Sensor II is 180º from Hall Sensor I on the same plate. Here's a link to a post that also contains a link to a downloadable pdf of the BMW LE Jetronic Manual that explains how your Brick's engine is managed. It could put things in perspective.

Testing guidance is also available on the Tech Page here in the form of illustrated documents titled Electronic Troubleshooting The EFI and Testing The Hall Sensors.

Excellent, thanx!

slowandlow wrote:...I've also been told it could be the throttle position switch (next to the no 4 intake) since a bad switch could influence the engine performance....

For easier reference for forum members who may have advice, consider going to the Profile section of the forum and under Signature, add the year & model (& even the last 7 digits of your VIN) to your posts every time.

The TPS is simply an on/off switch on LE Jetronic K bikes like yours, unlike the later Motronic (16V) K100 & K1100 models where it's a sensor. Some folk get about with the thing unplugged for less abrupt throttle feel. They have a very low failure rate, but unplug it and see if the symptom persists.

Two Wheels Better wrote:The TPS is simply an on/off switch on LE Jetronic K bikes like yours, . . . unplug it and see if the symptom persists.

A good suggestion, and success would pretty much eliminate ancestral curse as an underlying condition.

BTW, when engine is running, if i press the start button the rpm is rising, when i let go the rpm returns down to idle. Is this normal (electrical controlled "choke" at start sequence), or is my start button broken?

slowandlow wrote:BTW, when engine is running, if i press the start button the rpm is rising, when i let go the rpm returns down to idle. Is this normal (electrical controlled "choke" at start sequence), or is my start button broken?

Think of it as a hidden booster rocket.

It's not recommended as a habit but it richens slightly the mixture.

firstle wrote:really ? can this be done without damage to the motor 

If a corporate reference would help, here's a slightly fuzzy extract from p.16 of BMW's LE Jetronic training manual.

If engine speed increases when you press the start button it indicates that the idle mixture is lean.  This is normally caused by air leaks downstream of the air flow sensor. Although you seem to be fixated on ignition/electrical issues, I, personally, would spend my time investigating air leaks.

500rpm ? i must adjust the tick over on my k100

Intermission.

Thanx, ill continue my investigations...

Well that was easy, i did the gas can spray test and sure enough, i got a nice rise of the idle... It looks like all 4 upper and lower rubber tubes/ manifolds are bad... Ill also check those connectors for wear and tear. BTW, how do i remove the connector on the Throttle position switch? It has a metal clip around, do i just pull upwards until it releases or should the clip first be removed? Thanx!

You can try just pulling it off, but it may resist. In this case 'carefully does it'  because you do NOT want to lose/break that clip. Look at it carefully - both short sides fit into a shallow slot. Use a small screwdriver (a so-called 'jeweller's screwdriver') to lever out the side nearest to you. As you lever it out, the clip will start to rotate so make sure your other hand is in a position to stop it from pinging off. You only need one side out before you should be able to remove the TPS housing. As soon as the housing is off, push the clip back into the slot. The housing will push back on with the clip in place.

If you ever need to remove the housings from the fuel injectors or the thermostat housing (behind the radiator), use the same procedure.

If you do lose the clip, new ones are available from any specialist electronics shop or ebay. Ask for a clip for a 'three-way junior timer housing'. For the fuel injectors, they are a 'two-way junior timer housing'.

If the manifold and inlet rubbers are toast, you will also need to replace the crankcase breather, the vacuum caps and the vacuum tube on no.4 cylinder. Talk to Motorworks about secondhand manifold and inlet rubbers, but buy new vacuum caps, vacuum tube and crankcase breather.

When refitting the manifold and inlet rubbers, you have two choices. You can either buy new oetiker clamps and the tool to fit them, or go for 8mm-wide worm drives (which is what most of us have done - again, talk to Motorworks). If you use worm drives then buy stainless steel ones because you don't want to find that you need to remove the throttle bodies in future and discover that the worm drives have rusted solid. Be VERY careful when positioning the worm drives for cylinders 2 and 3 as it is very easy to foul the throttle bodies and jam the throttle open (or have it not open at all).

Great, thanx alot! ( I can remove only the connector (without removing the TPS housing) just to check the pins, right?)

Yes.