Kev, your o2 mod looks the way to go - I'll definitely invest soon (saw your ebay listing).
The mechanic had a go at sorting the surging and did a pretty good job .... BUT, over time I realised it was at the expense of the top end of the rev range (harsh - esp freeway). He confirmed that he only played with the CO setting and now wants $85 dollars to change it back - it's been awhile, so I can't really argue too much - nor do I want to (argue or go back to him - other issues too).
I went into the CO setting and it was on 30, which seems pretty high from what I've read here. I changed it back to 20 and gave it a ride - the surge is marginally (?) worse, but the higher revs are noticeably better. The issue is I don't know what the original setting was. I'll try to get the original settings off him, but in case I can't .... from what I gather in the forum, 8-10 is the normal OEM setting. Is that about right?
I know it's different between bikes, but it can't be too different. FYI, bike is in Melb, Oz.

OK, rang mechanic, and he says they wouldn't have recorded it (!!) and it is usually 0 (zero) "on those bikes" what ever that means .. blah, blah ... obviously no real recollection, and my $85 session will be an episode of trial and error.
Right so, not going back to him.
Can anyone who has changed their CO1 levels, report back on what their original settings were, and maybe I can get a bit of a consensus/average and go from there?
Mine is a >2007 xtx if that makes a difference (2009 - with O2 sensor).

It would be Zero to +5 on a 2007 model, if they charged you $85 to adjust the Co they are a rip off, mechanic's like these guys give us a bad reputation.

The Co adjustment makes no A/F corrections over a TPS reading of 18 which is around 2% throttle opening, after a TPS angle of 18 or 2% throttle opening the bike runs on a pre-programmed fuel map which can only be adjusted with one of my fuel mods or Power Commander + one of my O2 Controllers or Dynojets O2 Optimizer.

Would that be the same for 2009?
.... sorry, I wasn't very clear - I was saying it's a post2007 and had the actual year in brackets - it's a 2009 (it's got the 5VK-8591A-11 ECU).
He/they also broke my left side tank cowl snap-mount - granted it was a tough ****** to slip out of the grommet, but I previously managed it without breaking it - they tried to glue it back with Tarzan's grip (or snot?)!! I promptly dropped the bike on it about a week later (broke it proper) so it was a moot point. But overall, very unimpressed.
Like I said, I took the CO1 back to 20 and the engine is revving much more freely now (again) - much nicer to ride. By the sound of it, I could wind it back even more (and I will try it). It seems pretty good - and the snatchy low end is coming back.
I'll play with the TPS angle a bit too. I also get the cut-out - usually when I'm blipping the throttle for a down-shift.

As mention the Co1 adjustment is basicaly from idle to 2% throttle & can not adjust the A/F ratio above 2% throttle openings.

I would set your Co1 to 5 to 10, a CO set above 20 on your bike will be to rich at part throttle & will cause the bike to cut out as mentioned.

The only way to reduce the surging is to control the open & closed loop fuellings, the Co1 adjustment can not adjust these areas.

Open loop with a Power Commander or one of my fuel mods.
Closed loop with a O2 optimizer or one of my O2 controller.

Yep, after reading up in here about open vs closed loop (and why Yamaha do it) I see why the gizmo is needed.
I definitely will wind the CO1 back some more. Riding today confirmed that it's better at high revs, but nowhere near as free spinning as it used to be. I'll live with the surging until I get the gizmo (O2 Optimiser) - I much prefer to be able to spin up the engine when I need to.
It really was nasty at high revs on the CO1=30 setting - actually, to be accurate, it was more like steady throttle at high revs (freeway etc) - although surging at low revs was pretty much gone.
Thanks for the advice - although I am a bit confused ... Are there 3 (Kev) mod options? - I thought that there were two:
1) Fuel Mod: (from 660.com store or self-build) - change air temp reading - richens A:F for more power and/or for intake/exhaust mods.
2) O2 Optimizer: (from eBay) - changes O2 reading - to control surging (makes closed loop A:F closer to open loop A:F)
3?) O2 Controller: (?) - superseded by O2 Optimizer?

I only sell two mods that controls the A/F ratio.

A fuel mod.

http://www.xt660.com/site/node/136

A O2 controller.

Can be bought via PM or off Ebay.

http://www.ebay.com.au/itm/130788019598?ssPageName=STRK:MESELX:IT&_trksid=p39 84.m1555.l2649

Dynojet sell a O2 Optimizer but it can only be bought with a PCV Power commander.

http://www.off-the-road.de/XT-660/XT-Power/Engine-Tuning/Intake/Power-Commander-XT660R-X-O2-Optimizer.html

Oh, then it was the O2 Controller I thought I was describing.
I think the dynojet stuff is a bit over priced, and you do good work here - and at ktmsmt.com (I'm thinking of the SMT as my next bike).
I was also thinking of trying the EJK EFI controller: http://www.kenma.com.au/electronic_jet_kit.html - interesting claims, and the fact that it is real world tuning (no dyno) and the price.

As you know from the SMT forum I do real live tuning on my SMR using Dynojets equipement then import the PCV map directly into the KTM's ECU, change the rev limiter turn off the O2 sensors & a whole lot of other stuff, this type of tuning is the future.

Live tuning can be done on the XT660 & the equipment is available I was persuing this avenue until I sold my XTX, in the end I could not go any further as Yamaha Australia stopped importuing the XTX & I wanted a new bike, I love my Motards & have moved on to a SMR.

This is the programmer that was used on the very first XTX ECU reflash where the rev limiter was raised.

http://www.aliexpress.com/item/Hotsale-Piasini-Serial-Suite-v4-1-master/566972809.html

Anyway we are getting a bit off track on the O2 sensor thread.

Kev, you are a wizard. The bike is excellent - standard intake/exhaust and still makes a huge difference - especially the snatching/surging. I'm singing your praise in the XT660 facebook group - hopefully you get some new customers.
Tax man likes me, so DNA 1 and 2 and I'll drill the airbox out.
Edit: For anyone interested ... I bought (from Kev) both the Fuel mod (set to 1 o'clock for now - because no air/exhaust mods yet) and the O2 Controller (see picture - I didn't want to impede the fan and ... OK, I was bored, so fabricated a mounting bracket for it).