(actually, the base for this was originally pulled by some user who took it from David Saxton's article on the internet's best TDM site: Carpe-TDM.
I rephrased and added some to the original text to update it to current experience - Marc
REGULATION OF THE AIR VOLUME OF THE EXHAUST GASES
NOTE: To set the density of carbon monoxide (CO) in the exhaust gas.
(translated into common tuning terms, this means that you are adjusting the fuel injection to increase or decrease the amount of fuel delivered. Marc)
To activate modes "CO" and "DIAG":
Press together the "SELECT" and "RESET" keys, and turn the ignition switch on "ON".
(NOTE: It takes ~eight seconds after turning on the ignition switch - until "DIAG" appears on the dash display.)
In order to get to the "CO" mode: (so you can adjust fuel delivery)
Push "SELECT" in order to toggle the display between the "DIAG" mode to the "CO" mode, and vice versa.
Push together the "SELECT" and "RESET" keys for two seconds and the word "C1" should appear on the display.
(On the 900, C1 means the left side cylinder 1)
In order to select the cylinder to tune:
Press "SELECT" or "RESET" in order to select the cylinder (C1 or C2). Toggle between each to select the cylinder required.
When the required cylinder number is showing on the display ("C1" or "C2") -
Press together the "SELECT" and "RESET" keys for two seconds and the current value for the selected cylinder will be displayed (eg. "9").
Write down current values for each cylinder so you know where you started !!
To change the volume of carbon monoxide in the exhaust gas: (translate: to adjust fuel for best running and best power....)
On the display is shown a figure between -128 and 128 where -128 is very lean and 128 is very rich.
Press "RESET" button in order to lower the numerical value and decrease the amount of fuel delivered to LEAN the mixture.
Press "SELECT" in order to increase the numeric value and increase the amount of fuel delivererd / RICHEN the mixture.
Press together the "SELECT' and " RESET" keys in order to return to the selected cylinder number ("C1" or "C2"),
and go back to 3 to tune the other cylinder.
To SAVE the settings, turn "OFF" the ignition switch.
Now - all that being said.... Additional Key points -
When you are pushing buttons and increasing or decreasing the C1 or C2 numbers, you can hear the engine change exhaust note, showing that he fuel delivery is changing as your are changing the number values.
That's called "REALTIME" fuel changes.
If you have a dyno like an EC997, you can watch the engine power smoothness (Flux Value) change as you change the fuel rates at "cruise" throttle and rpm levels.
The better you have the mixture, the smoother the engine will be. (but we all knew that).
Also - the 900 uses c1 for the cylinder 1 (left) and c2 for cylinder 2 (right).
That's not necessarily true for other Yamaha's. The "kit" 600 and 1000 ECU's have tuning "ranges", based on throttle positions and rpms.
The street Yamaha's, R6, R1, Warrior all tune similarly to this, but you need to know the "secret" handshake to get into "CO" mode......