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Old 04-28-2010, 05:38 PM   #14
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JoelWisman's Avatar
Joined: Apr 2009
Location: STL, MO, USA
Oddometer: 1,392
Originally Posted by itsatdm
Sounds like this is the reason for the the slight stumble, when slowly trying to ease over things just off idle. If it is, than doesn't sound like there is much that can be done about it.
(quoting what I said about no fuel adaption at idle)

actually what I said was much to simplified.

There are three types of adaption, "additive", "multiplicative" and "extrapolated".

Addative adaption occurs on the fly as soon as the engine is warm. The F800GS uses a standard narrow band slow oxygen sensor and it is too far back in the exhaust to be useful without causing idle to hunt and peck all over the place. Addative adaption occurs once the sensor is warm at just above idle all the way to just short of wide open throttle (wot).

Multiplicicative adaption occurs over time and is remembered and incorporated as a conversion factor of the base fuel map. Multiplicitave is remembered from start up cycle to start up cycle and changes slowly over miles and large fractions of an hour of running.

The F800GS does use multiplicative adaption for idle fuel mix as well as part throttle fuel mix.

Lastly there is extrapolated adaption. The F800GS's O2 sensor is not capable of measuring fuel mixtures that are significantly different then lambda, and yet for a variaty of reasons INCLUDING emissions you want a pretty rich mixture at WOT. As such, the F800GS engine management computer enriches the mixture at near full throttle through WOT by a certain percent. This enrichment is based on the modified multiplicitive fuel map and is therefore called extropolated adaption.

The idle fuel mix is adapted but not well, it takes it some time to be optimal when the energy or oxidizer content of fuel changes. When I have gone from oxygenated to unoxygenated fuel it took about 20 minutes for the o2 sensor to start crossing.

The real reason for jerky transitions from idle is a lack of acurate sensors such as an air mass sensor or at least MAP sensor and the fact that BMW has so far failed or does not wish to damp this transition with industry techniques.

My guess is that the snapy throttle sells bikes after a test ride to typical F800GS customer, even though it is sub optimal off road in technical stuff.

As for stumbles going from part throttle to idle. This shouldn't happen if everything has had time to adapt to fuel and conditions but does on some bikes and I have no idea why.
Owned to date. Honda Aero 50, Honda Elite 80, Honda Elite 250x2, Suzuki Katana, Suzuki RF600, Yamaha YZF1000R, Kymco Xciting 500, Suzuki GS500, Suzuki Burgman 650, BMW F800GSx2, BMW S1000RR, Aprilia Scarabeo 200, Aprilia Caponord, Aprilia Sportcity 250
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