But the other half of the equation is fuel balance. This is easy to ignore because there is no way to easily measure it, and nothing to adjust. And it is wrong to believe that because the injector pulses are equal, that the amount of fuel or the quality of its atomization is equal.
At the speeds where surging is often experienced, throttle angles are low leading easily to air imbalance. But the injector pulse widths are also small, on the order of 2/1000's of a second. Of that 2mS, half of the time is the time to open the injector and the time to close the injector. So 1 mS is Open/Close and 1mS is spraying fuel. Of the 1 mS when the injector is spraying fuel, there can be different rates and different atomization pattern due to slight fouling and orifice sizes. In addition, as injectors age the open/close, or "dead time" as it's known, changes. This can easily lead to significant differences in fuel L/R and therefore power.
The notion in the prior post that combustion can take place between 7:1 and 19:1 is true, but wrong-minded. If the difference between cylinders was merely (let alone 7:1 or 19:1) 13.2:1 (roughly best power mixture) and 16.4:1 (roughly best economy), a twin cylinder motorcycle would be unrideable. But that gets to my real point which is why adding 4-8% more fuel that a narrowband sensor dictates makes such a significant difference.
In simple terms, you can get the power between cylinders to be close to equal by air balancing alone, IF there is enough fuel in the mixture to consume all the available oxygen. Once there is enough fuel to consume all the O2, then a bit more fuel in one cylinder doesn't effect the power balance L/R. The other thing to do would be to get perfectly equal fuel side to side and then add enough air so that all the fuel was consumed.
There are no adjustments to make fuel equal side to side so we have to balance air side to side and then, for a really smooth running motorcycle, add enough fuel so that all the O2 is consumed. The stock mixture is 14.7:1 and it turns out you only need to add 4-8% more fuel (given typical injector mismatch) to consume all the unburned air. So although 4% doesn't sound like much, it very often does the job. And the best part, of the 4% fuel that gets added (or 8% if you like) half of it is converted to power so not much is wasted.
The last consideration is the O2 sensor itself. The prior post mentions that there is only a TPS on one of the two cylinders. It should be noted that there is only one O2 sensor for both cylinders. Therefore if you perfectly balance the air but the L/R fuel is imbalanced by 6%, one cylinder is running at 14.4:1 and the other is running at 15.0:1 this too is a large imbalance. The greater the injector mismatch, the more additional fuel you need to add, hence my estimate of a 4-8% range.
So that's the rest of the story. Graphs and details in this admittedly long thread: http://forums.bmwmoa.org/showthread....and-O2-Sensors