Thx for replies. Iv spent more time working on this bike than actually enjoying the feel of it on the trails.
My feeling is that it?s the ignition coil breaking down. I?m sure the earth of everything?s ng is fine as I had been chasing a spark issue for ages so I have cleaned/replaced all connections and earth attaching locations. Befriend re the stator was replaced it wasn?t breaking down at revs but now tha the stator is really pumping voltage out I guess the ignition coil could be failing? Any chance the CDI would be failing at high revs? I?ll be replacing the ignition coil in the morning so hopefully is going t fix the issue and then can ride the beast.

So far I?ve rebuilt forks, shock, new barrel, new S3 high comp head, piston/rings, new boysen carbon fibre reeds, brakes and the stator. As spares I have an ignition coil and a cdi based on the RM I think as when it was installed it didn?t want to run at all just backfire. They need to be retarded by about 6degrees according to reading this forum.

You may want to double check you spark plug gap and make sure it is not too wide.

ok changed coil and lead, still the same issues.
spark gap is 0.74mm br8es brand new
carb is pwk38 pilot 38, main 178, needle n1ef clip2
new stator this 1 https://www.ebay.com.au/itm/KTM-EXC1...72.m2749.l2649
S3 head with black insert
new S3 barrel about 10hrs
new piton/rings about 10hrs
powervalve is working well,from visual can see the arm push to full open whislt reving engine. and stays full open with this breakdown issue
all earths are good as i was previously chasing a sparking issue that ended up being the stator, tested to spec just wasnt pushing a good spark for some reason. bike wouldnt start then it would, then fail again. ended up changing stator and fired 1st kick as it has since, but now has this breaking down issue

vid is idle slow up to wot held wot for about 5 seconds can see the fuel spitting out as it breaks down. then close throttle to idle and then open and kills it.
https://youtu.be/8MHZzGNPjmU

exhuast packing? i havent touched this. standard gg exp chamber with FMF turbine core silencer. just checked packing its actually a bit old but silencer is clear has same issue without silencer on.
could the timing be out slightly and that causing the issue? the original stator had the timing washer in and i placed that in the new stator. this topend breaking down issue has only started since the new stator. im hoping its not the stator as its new.
to me it doesnt feel/sound fuel as i can ride around low to mid fine but once revs get up high and powervalves open it starts to breakdown. can ride at 70kph with wot and doesnt breakdown till band kicks right in

edit running premium unleaded 98 fuel

Hows your coolant level?,i seem to recall someone on the forum having issues with a s3 head mod...but i cant recall what the problem was...

coolant level is fine and temps are fine as well, from hour meter with temp gauge. i know that this is not a great indicator for temps but its still an indicator

Watching this thread with interest as my 07 250 is almost exactly the same.
Only difference is mine will eventually "clear" and get past the spluttery stage and then rips as normal to full revs.
If I keep it at high revs it will stay there, but once back down to putter around speeds it will splutter again when trying to get back up in revs :-(

Matt your issue just sounds like the needle. Drop it down a position by raising it a clip and see if it gets better. Then search for a better needle. Stock is rubbish.

Cheers fella - I did change to NEDW to try and fix this, so will try adjusting clip.

(Sorry Yowie - didn't mean to hijack thread).

I run that needle now in my 300 but with a later AS2 carb. Was on top clip but touch lean going back one to trial. You requirements will vary.

Haha all good Matt, I pretty much changed the subject anyway. Hopefully I?ll be testing the beast again tomorrow arvo. I put a 170 MJ this n just to make sure it wasn?t running too rich and the power valve system had a failure. The little spigot for the actuator arm came out. So welded it up today and will be back on the bike tomorrow after work. Hopefully she rips.