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04 yz250f high idle. jet too big?

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Hi. A friend of mine just picked up a 04 yz250f. He got a good deal on it but it's not running quite right. It looks like it's been re-jetted because he's got the remains of a jetting kit but neither of us are very experienced with jetting ( I know a little) so are a bit unsure what the cause of the symptoms could be. Anyway basically the bike starts ok but revs quite high straight away. Sort of like half throttle. I tried using the fuel screw to make the revs hit their peak and then winding the revs down with the idle screw which I read is the way to set the fuel screw but the revs come down to a certain level and then if you turn it down anymore the bike just dies. That's got me thinking it's jetted wrong and when we turn the idle down too low were basically shutting the fuel off or something like that. If you ride the bike it's a little boggy sometimes (mainly when pulling away) but it still rips through the gears nicely with no sudden bogging halfway through a gear or anything like that. Can anyone with a bit more experience give me a few things to check and maybe some standard jet sizes to stick back in because as far as I can tell it's only got an aftermarket end can so i'm thinking standard jetting would probably be a good place to start.

Thanks for your help

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