K75 air flow meter

[K75Slark]

Theres a rubber plug on the top of the air box, removing this gives access to a hex screw on the air flow meter. What does it do, and whats the consequences if you twiddle with?

[Scott_]

It's my understanding that it adjusts the spring tension on the air damper in the flow meter assy.

As you adjust the damper tension it changes the amount of air allowed thru, and thus the meter output effects the jetronic fuel control, and that in-turn affects your fuel efficiency and exhaust CO output mixture.

I haven't touched one, but that's what I remember reading from others more knowledgeable on this than me......

[Inge K.]

It's correct that the screw changes the amount of air through and ekshaust CO content.

The function is very simple....at idle the air metering flap is closed but some unmetered air is
entering the engine via a bypass channel, the hex screw alters the cross sectional area of this channel.

[nortonbrian]

Is there a base setting or does it require a Co meter?

[Inge K.]

Seems to remember it was 4 turns out from fully seated, but not quite shure about this.

[wmax351]

You can do the "lean drop" method. Set the idle to 1000 rpm. Adjust the bypass screw to maximum rpm. Reset idle to 1000 rpm. Adjust to maximum rpm. Reset idle to 1000 rpm. Screw it out for a 50 rpm drop.

[grant71]

hello. i ve a 87 k100rs which had a sooty exhaust and used a lot of fuel. about 10 litres per 100 kms. on advice of an experienced BMW mechanic, i took the bike to a friendly carb shop andused his exhaust CO metre. turnedthe relevant screw anti clockwise until the metre reading was 2.5 ( it was 10 before i started) .my fuel consumption has dropped to about 7.5 lit per 100 km and the exhaust is slightly sooty.

[argent brick]

Hi Grant. I thought you were planning to sell your K bike.

[grant71]

Hello Lynn. the problem is i like it too much. like a good woman. he he

[TS87KLT]

Speaking from more than a little Bosch L-Jet experience -- f***ing with the AFM is the last thing you should do.  Or the thing after that.

If your engine is not running like it should, confirm no air leaks after the AFM, including injector o-rings, intake boots, vacuum lines, gaskets...  Confirm fuel pressure and volume are to spec.  Confirm consistent spark, and timing is correct.  Check compression.  Confirm valve clearance is in spec.  Confirm coolant temp sensor is working to spec.  Confirm the throttle position switch is adjusted correctly.  Confirm Hall sensor function and air gap.

If you deal with all of that and the engine still isn't running right, consider getting it rebuilt  Leave the damn AFM alone.   :soapbox:

OK, if you MUST futz with the AFM -- Confirm AFM air flap has full range of motion and no binding.  Clean the inside with carb cleaner, so there is no oil or soot in there.  Then, carefully pop off the plastic cover, and clean the wiper track with some DeOxit or just good ol' TV tuner spray from Rat-eeoh Shack.  Likewise the electrical plug, and put some dielectric grease in the plug contacts.  Then put the cover back on with some sili-snot, and put the AFM back on the engine.  Leave the damn air bypass screw alone!

[wmax351]

Speaking from more than a little Bosch L-Jet experience -- f***ing with the AFM is the last thing you should do.  Or the thing after that.

If your engine is not running like it should, confirm no air leaks after the AFM, including injector o-rings, intake boots, vacuum lines, gaskets...  Confirm fuel pressure and volume are to spec.  Confirm consistent spark, and timing is correct.  Check compression.  Confirm valve clearance is in spec.  Confirm coolant temp sensor is working to spec.  Confirm the throttle position switch is adjusted correctly.  Confirm Hall sensor function and air gap.

If you deal with all of that and the engine still isn't running right, consider getting it rebuilt  Leave the damn AFM alone.   :soapbox:

OK, if you MUST futz with the AFM -- Confirm AFM air flap has full range of motion and no binding.  Clean the inside with carb cleaner, so there is no oil or soot in there.  Then, carefully pop off the plastic cover, and clean the wiper track with some DeOxit or just good ol' TV tuner spray from Rat-eeoh Shack.  Likewise the electrical plug, and put some dielectric grease in the plug contacts.  Then put the cover back on with some sili-snot, and put the AFM back on the engine.  Leave the damn air bypass screw alone!

+1 on messing with anything inside.


Though the bypass screw is a regular adjustment, and can be made without a exhaust analyzer.

[TimTyler]

Here's the Rob Lentini method af adjusting the CO from http://skylands.ibmwr.org/tom/tech/co-setting.html

Here's a manual method of setting L-Jetronic K bike fuel injection CO (carbon monoxide) mixture without the need for an exhuast gas analyzer:

• Thoroughly warm the engine.
• If you live above 4000 ft elevation, be sure to install the high altitude compensating plug into the harness. There's a receptacle taped to the frame on the left under the side panel.
• Remove the rubber plug from the top right corner of the air box, above the engine.
  
• Using a 5mm allen, adjust the air flow meter idle air bypass screw under the plug for highest attainable idle speed. This should be about 1-2 turns out from the fully-seated position CW.
• Using a Carb Stix mercury manometer or vacuum gages, reset the butterfly bypass screws to resynchronize and establish an idle of 1050 rpm, or about one tach needle width above 1000 rpm.
• Now turn the idle air bypass screw CCW (from the top) until rpm is lowered by 50 rpm to 1000, about 4-5 total turns out. (This is called the lean drop method of setting CO, if you don't have an exhaust analyzer.)

As a final check, you know you have done it all correctly when:

 - You press the starter button with the engine running and the rpm stays the same or slightly increases (enriching signal to the computer).

 - You pull out the high altitude plug and the rpm also increases (again, enrichening signal to the computer, but less so than the starter. You've set it lean, so enrichening will increase rpm).

[AussieBrick]

Soooooo....sometimes you see photos of the throttle bodies with cone air filters and NO air-flow meter in sight. What is happening there?

[wmax351]

Soooooo....sometimes you see photos of the throttle bodies with cone air filters and NO air-flow meter in sight. What is happening there?

Those are the Motronic K bikes. They use an Alpha-N system (throttle position, RPM, temp, etc) instead of an airflow meter type fueling algorithm.

[AussieBrick]

351. Excellent. At a glance, is there an obvious give-away sign which bike is which?
Ie. Without knowing your Ks

[Inge K.]

All 2 valve models have Jetronic.
All 4 valve models have Motronic.
Your -93/LT have Motronic MA 2.1

[AussieBrick]

Aha. There you go. Very helpful /-).

[AussieBrick]

Inge, as I am new to these bikes and trying to work out what projects I am going to do I am trying to get to grips with this Jetronic/Motronic changeover point. I got this from wiki....

"""The LE Jetronic fuel injection system with separate ignition management was replaced by Motronic engine management with the introduction of four-valve cylinder heads with the 1990 K1 and the 1991 K100RS.[8]"""

So, being the dummy here, if a K bike has the 16valve logo on the tappet cover, is it Motronic regardles of being a K100 or K1100?

[Scott_]

Only so long as someone hasn't replaced the cover...........highly unlikely, but you never know.

[AussieBrick]

Any other giveaways? Eg. Flexible (corrugated) intake rubbers rather than short straight with airbox immediately above????

[Inge K.]

Any other giveaways? Eg. Flexible (corrugated) intake rubbers rather than short straight with airbox immediately above????

Correct, that's is one of the differences between 8/16V.