Urgent help required TC250 2010 !!

I need help urgently, have 1st round of Western Australia Championship coming up in 24 hours, it is my son's first race in the Pro- Lites, Bike went to dealer to fit new piston, received bike back and when we took it for a spin we had problems , when in neutral the bike idles very well, when you twist the throttle engine response good, no problem, but when driving the engine breakes down under load, Checked the usual, fitted new plug, cleaned carby, all ok, what can be wrong?
Cant bring bike back to dealer as he is to far away, I called him and he doesnt know what the problem is either, can anyone help?
What does the sensor near the gear lever do? Can it be the automatic decompressor?

I'd bet they put one of the cams in out of time.....maybe one tooth out....or maybe the valve clearance is too tight.

On my '10 TE250 the cam gears have marks on them that point together at TDC. I have not seen the manual.

Take off the valve cover and look at the marks and check the valve lash.

The good news is this is still an easy job on the new bikes.

FYG, the cam caps get 6nm on the small bolts and 10nm on the larger ones.

Thanks very much for your reply, the timing was ok, in the end it was just one of these things, in desperation I decided to remove the sparkplug cap, fitted it back on and the problem disappeared.
Cant explain but it worked, we are now set for tomorrow.

UR probably just a little over excited about the race Good luck with the race!

Good to hear that things are ok.
I know the guys that fixed the bike and they are sound.
I hope that you guys have a win GO husky.