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04 yz250f all of a sudden no start?

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Hey folks I have a 2004 yz250f I'm having issues with all of a sudden. Here's the deal. I did a top end rebuild. Adjusted valves to spec. New plug. I have about 3 rides on the bike since then. Bikes always started in 1 or 2 kicks.

So I had to replace the clutch and throttle cable last week. Bike sat for a week maybe 7 or 8 days total and now it won't start. It's getting spark. I took the carb apart and did a major clean. I tried to bump start it and it tried running but sounded like it was running with the exhaust plug in. Checked timing and valves again and all are good. Bike backfires every like 10 kicks too.

All work was done by a professional mechanic. Can anyone help me with this?

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Who did the rebuild?...it states you did but all work at first but then stated it was done by a processional mechanic at the end of the post...is the cam chain old and worn?? Are the cam gears worn down? Cam tensioner worn out?? Have you done a leak down test? If you replaced the piston did you make sure the piston rings are placed right and the ring splits placed at proper location?

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Sorry, a friend of mine owns a motorcycle shop around here. He did the tip end rebuild, valve adjustments, and timing. I did the clutch plate kit and throttle cable. I had a brand new cam chain installed with the top end because so many people recommended I do that. I also had a brand new cam chain tensioner put on by him because some friends were saying the stock ones are known to go bad. I haven't done a leakdown test on it yet. And I'm considering the piston rings are set right because I he does it for a living. And cam gears are still good.

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So here's an update.

Had a different mechanic look at bike. He looked in the carb first and said the accelerator pump diaphragm was shredded. So he replaced it. Bike started right up. Ran for almost a minute beautifully and then shut off. Won't start at all again.

Did a leakdown test which read 18%. He said there may have been a small leak he could hear through the air filter opening. Still can figured out why the bike won't start up anymore.

Can anyone please help me?

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