Think I experienced the famous surging on the XT today.
Wasnt too sure what to expect reading other posts concerning this issue,
To me it seems the bike is saying open the throttle and get going.

tell us.. what year ye have, what model. whats yer mileage, an what exactly is it doin.... am only askin as ye need tae feed us wi as much info ye are experiencing....

__________________
1st post of 2007, 2008, 2009 & 2014 gold star award

ok..tell us what ECU you have...to do this is very simple....

1. remove your seat
2. take your keys out of the side panel, and undo the top and bottom bolts on this side panel (you will need a 10mm socket).
3. Pull out the rear-most corner of your side-panel, so it comes out of the grommit.
4. now, slide your side-panel rearwards, so that the two "ears" come out of the petrol tank cover...and Voila!

Your ECU is the box nearest the petrol tank, just read off the numbers on the sticker. If you get stuck, have a look at the photos on this link....to show you what the ECU looks like, and the numbers needed.
http://www.xt660.com/showthread.php?t=8546

__________________
2008 XTR Mods : DNA Stage 3 Airbox, PC111, Carbon Cans Ovals, Steve Ruth built XTR734cc Big Bore with - High Desert Dyno (HDD)/ Cuervo Racing Gas Flowed / Ported Head fitted with +1mm Valves & uprated Springs, HDD 5050 Camshaft, HDD +3mm Throttle Body, CP 105.5mm 11:1 Piston, Custom Mapping, Piasini ECU, Renthal 613-01, Barkbusters VPS, Pivot Pegz, Touratech Bashplate, Foam Grips, Kev's O2 Sensor Mod, Kev's Extra Coolant Cooler, Wilbers Progressive Fork Springs and Rear Shock

Ok, The surging I am experiencing is at slow speed the bike feels it has a flat spot and a little hesitant but as soon as you blip the throttle a touch its fine, another example would be kind of fuel starvation.
But all in all I take no notice of it now I have done a couple of hundred miles on the bike.
I have looked at the numbers on the ECU as requested and they read 5VK-8591A-11. Hope this helps and look forward to your expert findings.
The bike is a 59 model xtx with 2000 on the clock.

Not up on my reg plate numbers but that sounds newer than the 04/05 models which had a faultly ECU. They where replaced by yamaha but some people did find you had to fight a bit to get the work done.

Anyway normally the way i describe the surging of the XTR is that at a certain set speed and gear its like yamaha (or the emissions police) have tried to make the bike cut the fuelling (as you say stravation) even though you might be asking the bike to do part throttle stuff. But the only thing is being a big single your part throttle and them trying to cut the fuel mean in stead of the engine feeling as if its dieing or straving of fuel and leaving you with an on off throttle theyve tried to get round that and it can jump where its like on off on off throttle even though your keeping the throttle the same. Basically like the engines trying to pick up and then lets go again.

Sometimes its not a problem as if your on a road no one else around you either speed up, slow down or flick another gear. But mid corner, traffic, or people are just travelling at that speed where it happens can be a bit of pain.

But as you said most riders have got use to it and ride round it, most with out even knowing after having the bike for a while.

Be careful as the same effects on the XT can be felt by having a loose chain or the cush drive rubbers being worn (wear out quiet quickly on the XT due to the torque aka wheelie fun).

__________________
Plough on

Yes Maxwell your description is pretty much spot on, and its true to say that once you have been riding the bike for a while you really dont take much notice.

Hi to all, I am a newbie to this forum so please be gentle!!

I currently have 5 Yams including a 2011 XT660X, and this is my third XT660, I luv em!! When I bought this one last summer it behaved perfectly for the first 4000 km and I really loved it, and I even told my dealer how much improved the engine management felt over my 2 previous ones (07 and 08). I had previously assumed that the surging was standard for a big single.......

BUT I recently noticed how poor the new one had become and it felt almost worse than the older ones had ever felt, surging all over the place and really tricky to ride in traffic! The flexibility had gone to pot and it would not cleanly pull 5th below 110 kph!! So I changed the spark plug which had very little effect, and then having read your forum I took the CO up by 10 points.

AMAZING!!! The bike will now pull top cleanly from 75kph like it used to, and has returned to being as sweet as it ever was. Thanks guys!! Simon