A fueling device offers an increase in A/F ratio across the whole rev range & does make the bike smoother through the whole rev range.

Not all XT660's are equal we have 5 different levels of firmware, all with different tune states. You can read about them here.

http://www.xt660.com/showthread.php?t=8546

As for your bike a 2009 model with O2 sensor the only way to get it to run smoothly low down or on the cruise is with a Power Commander + their O2 Emulator or my Fuel mod + my O2 Eliminator.

I no longer make my O2 eliminators as no one was buying them, so like some of the past XT660 mods I used to make I have stopped making the O2 Eliminators, I do have one left but it has been reserved until Sunday night 29/1 then it will up for sale again if not bought.

These XT660's do not like a A/F ratio of 14.7:1 where Yamaha have programed the closed loop A/F to. If you want a smooth XT660 ride with good fuel consumption you need the A/F ratio set to 13.6:1 in the open & closed loop. If you want to chase the best HP & Torque the A/F ratio needs to be set to 13.2:1, facts all Dyno proven.

An A/F ratio of 14.7:1 has nothing to do with a good running motor at all, it is set there purely for emission control & does not offer the best fuel consumption at that A/F ratio due to the fact you have reduced HP & Torque.

What I'd forgotten,want to know how well it runs??,never use first gear,it'll pull 2nd from the lights and still start to pick the front wheel up,do folks!! bikers in particulary get really upset?? oh yes!! with long legs from a standstill!!!