4v Engine Fuel/Air adjustment

[The Mighty Gryphon]

I have not been able to find an answer anywhere so I'm asking.  My K100RS 4V runs a bit rich as witnessed by fluffy black carbon deposits on the plugs.  The engine now has over 114,000 miles on it and I suspect that the injector orifices may be worn and flowing a bit more fuel than when they were new. 

The bike is not running the OEM exhaust , but an older K100 exhaust with the individual header pipes, so it doesn't have the oxygen sensor but instead has the screw thingy on the side of the relay box.

I have tried doing the adjustment of the idle screw on the relay box using the lean drop but it doesn't seem to help the rich situation despite making the engine run smoother and possibly stronger. 

My question is regarding the throttle position sensor which is supposed to send a signal to the Motronic unit indicating throttle position, and thus, air flow.  I have adjusted it to the voltage specified at the closed position and still get a rich mixture.  Is there any reason I shouldn't try leaning out the mixture by offsetting the sensor slightly to fool the Motronic into thinking there is less air so it injects less fuel? 

Side Note: Despite having good compression readings the engine does burn a slight amount of oil, about a quart every 800 to 1,000 miles, which I understand to be within the factory limits.  While this is quite a bit more than my two K75's I don't think it is enough to account for the carbon deposits on the plugs.

[kennybobby]

i suppose you checked the vacuum line to the FPR--if it is leaking it raises the fuel pressure and you end up with a richer mixture.

Lack of an oxygen sensor on Motronic will cause it to default to a richer mixture.  i once built a variable voltage O2 sensor simulator for testing on Motronic systems.  The controller is looking for a voltage in the right range, and a voltage that is oscillating back and forth from lean to rich in a given time period.  If those conditions are not met then it defaults to a richer mixture.

[The Mighty Gryphon]

FPR vacuum line is good. 

From what I can learn the pot on the relay box is supposed to compensate for the Oxygen sensor being removed.  I guess it is possible the pot is bad since I am doing an alternative adjustment procedure as I have no access to an exhaust gas analyzer without also buying $100 worth of time on a dyno.  Haven't seen any tests to check the operation of the pot.  I guess it's possible that I have a bad one.

Still, I wonder if the TP sensor can be used to trick the Motronic into running a little leaner and if that will cause any other problems. 

[Inge K.]

When you did bin the O2 sensor and installed the pot. meter....did you also change out the engine wiring harness and the Motronic unit?
Have you checked the air temp sensor (and its connector)?

[Martin]

Carbon deposits indicate it's running rich, so it's definitely a fuel problem not oil.
Regards Martin.

[The Mighty Gryphon]

Inge, the stock exhaust and the oxygen sensor were binned by a previous owner,possibly through a dealer some time some time in the mid 90's when they gave up on cracking header pipes.  I was told there were 3 warranty claims on cracked pipes on this bike before a Remus exhaust was installed. 

It appears the harness was changed when the pot was installed.  There is no sign of a connector for the oxygen sensor anywhere, not even cut wires.  How does one identify the correct Motronic unit? 

The bike has run rich since I  got it two years ago.  I removed the Remus exhaust and replaced it with a used K100 system because the Remus was embarrassingly loud.  The bike runs very strong and gets about 45 mpg.  I could live with the black plugs if I had to, but being anal about the tuning I'd really like to see proper color on the plugs and get rid of the rich smell of the exhaust.

[Inge K.]

How does one identify the correct Motronic unit? 

Cat models ECU have a green label with Bosch #0 61 200 270.
Non cat models have a blue or orange label with Bosch #0 61 200 250.

[The Mighty Gryphon]

ECU label is orange and has number 0 261 200 250. 

Inge, do you think it would be possible to adjust the TPS to "trick" the ECU into thinking there is less air slightly lean out the mixture, or would it be better to make the adjustment at the potentiometer?

[Inge K.]

Haven't done any experiment with that...so I don't know.
But one can read around in the different forums....isn't the TPS properly adjusted, the bike runs like shit.
The pot meter I would guess mostly affect the area around idle.

I would rather have checked the temp sensors and the related wiring.....rich mix points to too much
resistance in one (or both) of the circuits.

[The Mighty Gryphon]

I suppose that it wouldn't hurt to double check the operation and wiring of the temperature sensor.  Due to the hassle of getting to it, it will probably be a couple weeks before I can get to it.

Stay tuned...