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05 yz250F problem

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I have a 2005 yz250F and I have a problem with it, hopefully someone can help. I live in Northern IL, the temp here right now is around 80-90 degrees F. its about 60 -70 percent humidity. Anyway, heres my problem, when I am accelerating my bike is misfiring real bad, and has no power, and it bacfires and pops a lot on decel also. It is also really hard to start when cold. The wierd thing is though that when it was misfiring on accel, it went away after about 4-5 laps?? But it still popped and backfired on decel. but the problem keeps coming back on every ride, it is misfiring, and backfiring real bad. it just started to do this about 3 weeks ago, otherwise it ran perfect for 4 months. I have recenty cleaned and blew out all of the jets, air passages, and the whole carb, no dirt. checked my valves, those are also in spec. i am also putting amsoil ip in my gas to try and clean my system out, ive already used one tank full of gas with ip in it. i never use gas older than a week old, I also only buy the best gas i can get, BP, Shell, Mobile. Citgo and other lower end gas stations that are cheaper don't have the cleaning detergents needed, thats why they are cheaper. Maybe its the jetting? any suggestions on what I should do?? hopefully i can have this fixed by this weekend.

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what octane rating are you running in your bike generally in the new four strokes you need to run higher octane ratings. Also do you have an after market exhaust. If you do generally if you do not upgrade your jetting with the exhaust you will have this problem. Try going one size up in your main jet.

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Have you done anything that could have changed the timing lately?

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I am running 93 octane, I've been told that a stock 4 stroke motor won't run any better on race gas. And I've only had the timing covers off to get TDC when i checked my valves, I didn't change anything though. I think I'm going to try adjusting the fuel screw 1/4 turn richer, and if that doesn't work then I'll go up one on the main. Thanks for ideas, keep em comin.

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