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1990 K75 question

nickrides

Nick Kennedy
Hi Gents
While getting tires mounted recently my mechanic told me the following:
During cold post start up, you can get the engine to run in a enriched state by holding the starter button down for 4 seconds AFTER start up. I looked in my K75 book and there was no mention of this.. Is this correct?
Nick Kennedy
 
Depressing the starter button does enrichen the mixture. I have never heard that it holds that enrichment after the button is released. If he meant it would be richer while you held the button for 4 seconds that is correct. If he meant it would stay richer after that, it would be an undocumented feature I've never heard of.
 
Hi Gents
While getting tires mounted recently my mechanic told me the following:
During cold post start up, you can get the engine to run in a enriched state by holding the starter button down for 4 seconds AFTER start up. I looked in my K75 book and there was no mention of this.. Is this correct?
Nick Kennedy

Why would you need it to be richer after startup?
 
Use the "choke" on the bike to start when cold. Ride immediately and close choke as soon as possible. Paraphrased from owner's manual, page 42.
 
Bosch LE Jetronic has a start up enrichment feature built that is activated while the starter button is activated. That is the only time you need it. There is another idle enrichment feature built into these bikes. In the Throttle Position Switch (TPS) mounted to the rear of the Throttle Body (TB) rack there are two contacts. One set of contacts is engaged only when the throttle is released, or zero throttle, and the other set of contacts is engaged at full throttle. This is only true of L-Jet. Motronic requires a completely different type of TPS that measures throttle position througout the range of throttle motion.
At idle when the zero throttle contacts are engaged the idle mixture is enriched. As soon as you crack the throttle open idle enrichment ceases, and this is a good thing because if you ever played with one of these on a dyno with a gas analyzer with the TPS disconnected you will see that overswing of the vane in the air flow meter causes the mixture to richen up very quickly off idle. With the TPS connected there is a smooth transition from idle enrichement to air flow meter overswing enrichement, with the latter acting almost like the enrichement jet in a carb.
If you are above 2000 rpm and the throttle is closed, that same set of contacts tells the FI computer to shut off fuel flow as a way to improve gas mileage. The other set of contacts activate the full throttle enrichment feature, fattening up the mixture when you whack it open. Again, if you dyno one of these bikes with the TPS disconnected you see mixture go lean above 6000 rpm, which this feature corrects.
L-Jet was such a nice system, I wish BMW had kept it. It is analog and has response very much like a well sorted carb, much nicer than the gritty feel of Motronic.

Btw, the "choke" lever does not activate any sort of start up enrichment. What it does is open the throttle plates a bit while not disturbing the TPS so you maintain idle enrichment while availing yourself a higher idle speed. If you close the "choke" lever and use the throttle, you come off the zero throttle contact and lose idle enrichment. L-Jet was such a well sorted system. Also, if you put one on a dyno and look at the air fuel ratio across the RPM range it is a nice flat line, real thing of beauty. You have to spend so much dyno time (expensive and tedious) to get the same result out of so many digital fuel management systems. With L-Jet you can add exhausts or larger throttle bodies, adjust the spring on the air flow meter vane and the potentiometer and still you get this nice flat line for the AFR. I love L-Jet!
 
42906
I've also got a 1978 BMW 633 CSI with that same L-Jetronic F.I. system
Your right, that is a great analog system that really works and is simple to work on and trouble shoot. The only thing I wish it had was a oxygen sensor.
Nick Kennedy
 
42906
I've also got a 1978 BMW 633 CSI with that same L-Jetronic F.I. system
Your right, that is a great analog system that really works and is simple to work on and trouble shoot. The only thing I wish it had was a oxygen sensor.
Nick Kennedy

The main thing I see as a limitation the the L-Jetronic design is that will age the spring for the airflow meter waste gate can lose tension which allows the gate to open further than it should which creates a too-rich mixture at speed. There's no off-idle mixture adjustment as such without tearing the airflow meter apart and manually trying to recalibrate the spring tension. I know some folks who have done it, but it's apparently not an easy fix. However, it is a good opportunity to clean the contacts to get a more accurate signal of the gate position.
 
This reminds me that there is a really great book out there called
" How to Understand, Service and Modify Bosch Fuel Injection & Engine Management"
By Charles O. Probst SAE

It is a Idiots guide to L- Jetronic and LH-Jetronic systems [and all other Bosch FI systems] that is written for the layman to use. Fantastic book loaded with pictures and easy to understand text. Excellent explanations on how the system works through its various phases and how to test all the sensors. Perfect for a idiot like me!
I bought it through Bavarian Autosport in NH. to work on my 633 CSI and it covers the FI system in my 1990 K75.
Great reading, this one.
Nick Kennedy
 
This reminds me that there is a really great book out there called
" How to Understand, Service and Modify Bosch Fuel Injection & Engine Management"
By Charles O. Probst SAE

It is a Idiots guide to L- Jetronic and LH-Jetronic systems [and all other Bosch FI systems] that is written for the layman to use. Fantastic book loaded with pictures and easy to understand text. Excellent explanations on how the system works through its various phases and how to test all the sensors. Perfect for a idiot like me!
I bought it through Bavarian Autosport in NH. to work on my 633 CSI and it covers the FI system in my 1990 K75.
Great reading, this one.
Nick Kennedy


Thanks for the info on this book! Is this it?

http://www.amazon.com/Bosch-Fuel-Injection-Engine-Management/dp/0837603005
 
The main thing I see as a limitation the the L-Jetronic design is that will age the spring for the airflow meter waste gate can lose tension which allows the gate to open further than it should which creates a too-rich mixture at speed. There's no off-idle mixture adjustment as such without tearing the airflow meter apart and manually trying to recalibrate the spring tension. I know some folks who have done it, but it's apparently not an easy fix. However, it is a good opportunity to clean the contacts to get a more accurate signal of the gate position.

The most difficult part of the job is carefully prying off the black plastic cover. The potentiometer can be adjusted, but it is a very sensitive adjustment. Most of the time people do this to loosen the spring so the vane opens faster, improving throttle response. It is an effective modification and one I do on my bikes. The richer mixture from vane overswing acts like an accelerator pump in a carb.
When you put the cover back on use only clear RTV silicon sealant and only the thinnest bead. Use the applicator that comes with the tube and be patient and careful as you work.
 
The most difficult part of the job is carefully prying off the black plastic cover. The potentiometer can be adjusted, but it is a very sensitive adjustment. Most of the time people do this to loosen the spring so the vane opens faster, improving throttle response. It is an effective modification and one I do on my bikes. The richer mixture from vane overswing acts like an accelerator pump in a carb.
When you put the cover back on use only clear RTV silicon sealant and only the thinnest bead. Use the applicator that comes with the tube and be patient and careful as you work.

If you loosen (weaken) the spring, I can see getting more overswing, but won't the gate be further open than it should at all times, thus increasing the richness of the steady-state mixture?
 
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