Thanks for all the good replies!
I have of course opened the timing cap on the flywheel, so I'm positive that it's TDC. The bike have been served by a local shop and the bike runs good, but i don't got any other 250 4-stroke to compare with!
Yeah the bike is a little dirty but i only went I only went out for a quick ride. I live in Sweden and here it's below freezing, so I didn't want to wash it because it would take so long for the bike to dry! I always clean the bike after I been to the track!
MotoX178 and Aaron_Silidker:
Thanks! Yeah, I'm going to investigate the valves, but do you guys really think that all 4 valve seats are bad at the same time?
But right now it seems like the only possible solution.
I'll consider that. But I'll first try to sort out the problem and when i feel like there's nothing else to do I'll take it to the shop!
I think you mean the spark plug, it's really "deep down" on a 4-stroke.
I've talked to a guy and he said that it's probably my camshaft has turned in the camshaft sprocket (both inlet and exhaust), but i compared the control hole in the sprocket with controll hole in camshaft. So at least that ain't the problem! I have also turned the flywheel around with a friend trying to put in a thickness gauge between the camshaft and den valve lifter, but it just won't get in there!