K75RT Engine problem

[Grahame Coles]

I've had my 1995 K75RT for about 6 weeks. Its got 28,500 on the clock and I've put about 700 of those on it. Out at the weekend at the furthest point from home when it stalled at a junction and would only restart with full throttle and die if the revs dropped below 2000 or so...lots of smoke. Had to call out bike rescue.
Found a crack in the crankcase breather and replaced the hose with no improvement.The bike has done 200 miles since last service which included new plugs.
I've spent some time looking on the internet but can't find a similar problem.
The bike has been well cared for and had just returned from a 3000 mile trip round Europe when I bought it. I've ridden it carefully as this is the first bike I've owned..passed my test in 1969 on a scooter and not ridden on two wheels since!
Any advice greatly appreciated

[The Mighty Gryphon]

What do the spark plugs look like?  Take them out and mark them for the cylinder they were in to keep track of where they were.

Can you post some clear closeup photos of them? 

Any unusual noises when you crank the engine over?

Do you have access to a compression tester?  What is the compression in the cylinders?

Is there water in the engine oil?

Let's eliminate any catastrophic mechanical trauma as a cause before going any further.  I am suspecting a blown head gasket.  That is assuming the smoke is actually water vapor.

[Grahame Coles]

Many thanks, I'll get on it tomorrow and let you know what I find.
No unusual engine noises when running but did hear a high pitched metallic rattling noise about quarter of a mile before the bike stopped, thought something had fallen off the bike or I'd run over something.

[Grahame Coles]

I've checked the engine oil and it is not contaminated with water, the coolant is also oil free.
Have attached a photo of the spark plugs, as you can see they are all the same, black and sooty. These are the original plugs that came with the bike, I swapped them back just to eliminate the new plugs as a problem, there was no difference in running between the two sets.
When I swapped the original plugs during the service 200 miles ago they were all a perfect colour indicating healthy combustion.
I haven't got access to a compression tester at the moment

[pfls]

Check the coolant temperature sensor for a proper ohm reading. I can't remember for sure what it should read, but 960 ohms comes to mind. When it fails, you get a rich running condition, and the bike goes into a limp home mode. You may even see some gas dripping from the header/muffler joint.

[The Mighty Gryphon]

Sounds like things are ok mechanically.  Plugs are a dry sooty, is that correct? 

Does the high pitched rattle you mentioned occur when you crank the engine or when the engine runs now?  Or did it just occur before the problem started?

It looks like you have an overly rich fuel mixture.  I can think of four things that could cause that.

First, injectors are stuck open. It is unlikely to have dirt get past the fuel filter and get to all of them simultaneously.

Second, high fuel pressure caused by a malfunction in the fuel pressure regulator.  Possible, but unlikely.  You would need a pressure gauge capable of reading at least 65psi to check this. 

Third, bad ignition timing can give the appearance of a rich or lean mixture.  It's unlikely the timing changed as quickly as you describe the onset of the problem.

Fourth, the injectors are being told to go wide open by the Jetronic engine control module(ECU).  Injector output is calculated in the ECU with input from the above mentioned temperature sensor and the Airflow sensor.  I have no experience with either, so can offer no usable help.  Perhaps, others can chime in with suggestions for checking these items.

There are threads here discussing both of these devices.  A search should bring up some information on troubleshooting them.

[Grahame Coles]

The plugs were dry.
The rattle was a one off before the engine expired but I'm not 100% sure it was from the engine.
I'll take your advice and look into the ECU, airflow sensor etc.
Hope I can fix it before winter sets in as I was just relaxing and begining to enjoy the twisty bits.
Thanks for all your advice, much appreciated.

[TimTyler]

I'd try some Heet Water Remover (or similar) and a fresh fuel filter to start.

[Grahame Coles]

Thanks, I'll do that first as its easy and fairly cheap to do

[Freelancer]

A few possibilities, not yet mentioned, based upon your description.

Rattle and timing symptoms could indicate that the timing chain tensioner/guide may have self-destructed.

Plug wires are becoming a common culprit as these bikes age.

Coil?? Unlikely...tho....

Hmmm, when was last time heatsink compound/thermal paste was applied to that pesky ignition bit on the battery tray/shield?

Btw, What color of smoke are you getting? Too rich gas or burning oil?

Hope this helps,
Freelancer

[Grahame Coles]

Hi Freelancer
The smoke is light coloured and smells more like fuel than oil, but can't be 100% sure. A friend had a play with it last night, we took out the Air Flow Meter and it looked mechanically ok. The air filter was upside down and facing the wrong way so we fitted a new one. The engine runs on all 3 cylinders with the throttle well open and it will rev up ok but as soon as the revs drop below 3000 or so it dies.
We don't think it's a problem with the coils or plugs.
Your timing chain suggestion is interesting, would the engine run without rattling if the chain guide had failed as it is mechanically quiet.
If so the valve timing cold be out of sync.causing the poor running.
Not sure how to check this without stripping the engine.. a bit drastic if it's not the problem.
Grahame

[Inge K.]

Hmmm, when was last time heatsink compound/thermal paste was applied to that pesky ignition bit on the battery tray/Shield?

The ignition amp is only used on early Motronic (MA 2.1) euqipped models.........all K75's have Jetronic.

[Freelancer]

The engine runs on all 3 cylinders with the throttle well open and it will rev up ok but as soon as the revs drop below 3000 or so it dies.

Ding, Ding, Ding

Hi Grahame, I'm not ruling out other suggestions but the part I just quoted is setting off the old key symptom alarm.
Depending upon the answer to the next question, we should have a really good "guess".

Does the bike run pretty good above 3k rpm or is it really off/barely running?


I need to go back and read up but the
 "dies under 3k rpm" symptom points to the fuel enrichment circuit, if memory is correct. (?not sure of name/part?)
It'll come to me as soon as I stop thinking about it or I go back and dig through old threads.


Maybe RBM, Win max or one of the others with more K75/l-jetronic experience will know the exact problem and nomenclature.

Hope this helps,
Freelancer

[Freelancer]

Hmmm, when was last time heatsink compound/thermal paste was applied to that pesky ignition bit on the battery tray/Shield?

The ignition amp is only used on early Motronic (MA 2.1) euqipped models.........all K75's have Jetronic.

Whoops,
Sorry Grahame and thank you for correcting me Inge K.

Freelancer

[Grahame Coles]

Hi Freelancer

The engine runs without missing and is a bit sluggish,it also sounds and feels rougher than normal, definitely not a happy brick.
It won't start without full choke or the throttle at least half open open. If it doesn't start on the first attempt it sometimes backfires on the second attempt.

Grahame
 

[TimTyler]

My money is on an air leak somewhere and/or very tight exhaust valves.

[Grahame Coles]

Hi Tim

Had a good look round the air intake system yesterday (have replaced the split crankcase breather with no noticeable improvement) there is nothing obvious. The problem occurred suddenly with no deterioration beforehand, in fact I had just been thinking what a great bike it was....fatal!
I think I read that you can spray butane or some flammable gas around the connections while the engine is running and if you have a leak the engine speeds up, I'll look into doing that at the weekend.

Thanks
Grahame

[TimTyler]

OK. Not the valves then.

The bike has done 200 miles since last service which included new plugs.

Besides the plugs, what service was done?

[Freelancer]

My money is on an air leak somewhere and/or very tight exhaust valves.

I'm leaning away from this idea because of his black sooty plugs. Seems like an air leak would leave a lean read on the plugs.


Grahame- Another Question:

 How many miles on that tank of petrol?

What I'm getting at: "Is there any chance that you got some water logged ethanol?".

Just trying to remove simple solutions.

If you had recently filled up it could be bad gas, ...

If not, maybe you had a water bubble in the bottom of the tank.
Heck, one time I had an old gas cap gasket stop sealing and didn't find out 'til I rode through I heavy rain storm.


As to your question about the timing chain tensioner.

The only way I know of to check would be to remove the cover. Hence, why I'm trying to look for simpler fixes before recommending that you to tear into it.
And yes, the tensioner can fail and the chain not make a lot of noise afterward. It is rare but has happened before.
Still, let's look for simpler solutions first.

So my current recommended check list order:
Bad Petrol?
Clogged Fuel Filter?
Fuel pump or pumps prefilter?
Idle enrichment circuit?
TCS (Temperature Control Switch) Check connection?
L-Jetronic Ground?
L-Jetronic?
ICU/ICM- Ignition Control Unit?
Timing Chain/Tensioner?

Freelancer

[Grahame Coles]

OK will look at each in order of ease.
Re petrol, I topped up the tank before I set off, the tank was fairly full and only took about 5 litres, ran for 22 miles before breaking down.
Regarding the recent service:
Plugs, engine oil, final drive oil, gearbox oil, greased final driveshaft splines.
After problem changed air filter and crankcase breather hose

[Freelancer]

Ok, think it finally clicked.

When I say idle enrichment circuit, I'm actually referring to a couple of things. One of those things I've already mentioned...the TCS.

Another part is the TPS (?) Throttle Position Switch. Note- this is a switch as opposed to sensor.

The TPS stays closed until you open the throttle x amount. With x generally equaling about 3k rpm.

If this switch goes bad or vibrates loose you could have a constant open/on situation.

The Check for this should be pretty simple.
1) Visual check to make sure ithasn't come loose somehow.

2)You listen to it as you open the throttle. You should hear the switch click. (This can be done with the bike not running. You just want to hear the click.)

3)Visual inspection of the wire connections to the switch.

This is the thing I was trying to remember .

Btw, Somebody please correct me if my memory is wrong about the TPS and how it works.


Hope it is a simple/cheap fix,
Freelancer

[rbm]

The TPS, when correctly adjusted, will register fully closed or fully open on the throttle.  If you snap the throttle closed with the engine above 5000 RPM, the TPS tells the EFI to turn off the injectors until the RPMs dip below about 2000 RPM.  Once the RPMs decrease below 2000, the EFI turns the injectors back on.

To confirm the TPS might be the problem, disconnect it and ride the bike.  If the problem disappears, troubleshoot the TPS.  If the problem persists, it isn't the TPS.

Check out this K100-forum thread concerning the same problem.  The fix was replace the HES array.

[Freelancer]

The TPS, when correctly adjusted, will register fully closed or fully open on the throttle.  If you snap the throttle closed with the engine above 5000 RPM, the TPS tells the EFI to turn off the injectors until the RPMs dip below about 2000 RPM.  Once the RPMs decrease below 2000, the EFI turns the injectors back on.

To confirm the TPS might be the problem, disconnect it and ride the bike.  If the problem disappears, troubleshoot the TPS.  If the problem persists, it isn't the TPS.

Check out this K100-forum thread concerning the same problem.  The fix was replace the HES array.

Thank you for posting this. My bike has the Motronic setup so I'm not as familiar with how things work on the L-Jet. Just remembered someone having a similar problem that traced back to the TPS.

Quick question......
Could the HES make a rattling noise just before failure?


Asking because my gut keeps telling me that it sounds like a timing chain tensioner failure or something under the timing chain cover.
I haven't really pushed this opinion because checking would be the biggest hassle out of the various possibilities. Am hoping for the OP's sake that it is just bad fuel or cheap ground/switch fix.

Respectfully,
Freelancer

[K75RT Keith]

The HES sensors are riveted to the timing plat and likely wouldn't come loose.  If they've been replaced, they could be held in with screws and then maybe they could come loose.

Try finding your rattle by using a big screwdriver if you don't have a stethoscope.  Place the metal end against the running motor and the handle to your ear. check multiple areas to pin point the noise.

MAF,  If the flapper in the plenum that controls the airflow is malfunctioning or stuck, it will cause a rich mixture.  If you can reach it, then make sure it is clean and not covered in oil and that it moves.  There is an adjustment on the top cover to adjust the CO level.  To do that adjustment you'll need an exhaust gas analyzer.

When you replaced the air filter, you did check for debris in the plenum. Right?