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yz250f runs bad. i need your help!

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hi i have a 2003 yz 250f with a fmf exhaust on it when i bought it and i dont know if it was re jetted when i bought it or not. it idles great but if u rev it up a bit it cracks and back fires and sounds awful and i was wondering if it was the jetting or values. im thinking its the jetting because it still starts easy.

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A little backfire on deceleration is normal. A lot could be an air leak in the exhaust or your jetting is a little off. Try going a half turn in either direction on the fuel screw and see what happens. I would start by making it a little richer, that would be unscrewing the fuel screw. Be careful not to make it too rich or you'll have trouble hot starting it.

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hi i have a 2003 yz 250f with a fmf exhaust on it when i bought it and i dont know if it was re jetted when i bought it or not. it idles great but if u rev it up a bit it cracks and back fires and sounds awful and i was wondering if it was the jetting or values. im thinking its the jetting because it still starts easy.

If the PO installed that new FMF exhaust, he may not have installed new exhaust gaskets and could contribute to a leak causing some of the symptoms you describe. Worth ruling out.

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If the PO installed that new FMF exhaust, he may not have installed new exhaust gaskets and could contribute to a leak causing some of the symptoms you describe. Worth ruling out.

+1. That's my guess that he has an exhaust leak based on the description.

Either that or the fuel screw fell out or has a defective o ring on the FS.

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yea when i hold it steady at about half throttle it carries on lik that and if i accelerate it runs great and when i let off the gas thats when it starts to pop realy bad

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yea when i hold it steady at about half throttle it carries on lik that and if i accelerate it runs great and when i let off the gas thats when it starts to pop realy bad

Have you taken a look yet and eliminated the above possibilities? Hard to give much more advice until you can say what you've checked and eliminated.

Are you getting these symptoms with the bike on the stand, or while riding under load?

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when im riding the bike around if you have it half way open and just driving around it back fires contuinously. once you get on the throttle it clears up. i was trying to check the valve clearance on my bike because i had it all tore apart and was checking the carb which is all clean but a few of my valves were a tad tight. works at a dealership but he hasnt been able to be out to tell me if they need adjusted or not.

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