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04 YZ250f won't kick suddenly

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I have a yz250f. It's an 04 and i got it while it didn't work. I bought it from some guys that had a mechanic tell them that it wouldn't run because of the crankshaft. He said something about it was put in backwards? If that's even possible. My friend who knows a good bit about motorcycles and dirt bike engines since he made a Harley from scratch. He looked inside the bottom end and said the crank looks fine. The guys weren't bullshitting me because they gave me a brand new 200-something dollar crankshaft replacement kit. So they obviously thought it was the crankshaft. So my friend said it was top end problems. We took apart the top end, put it back together after checking the timing chain and valves. Before we did that it wouldn't kick. It was locked up. After putting it back together, it would kick but take a lot of kicks to start, usually it just floods the carbeurator and we have to wait. It would start up and run only choked then the bikes vibrations push the choke back and suddenly kill. So somehow the top end fixed itself. So now we went to shim the valves bc my friends 450 would take forever to kickstart, so he shimmed valves and it works now. So he shimmed mine. Here's where problems started. For some reason we decided to try to see if the engine would turn before we tightened the timing chain. So it was loose. It obviously locked up. So we were just like shit. So he tightened that, then tried again, it messed something up bc it still wasn't kicking. So we checked to see if it was still top dead center. Nope, so we top dead centered it and it still wouldn't turn. The plan for now is to check the crank today and take apart the top end and see if we find something and put it back together and see if it works. Maybe the loose chain chipped a gear and the chip is stuck somewhere important? Worst case scenario we have to actually replace the crank. And to note there's a valve or some part of a valve that is tight I forgot what he said it was. We are gonna have to shave it down and if that don't work buy another. But any idea on what it could be? I need help... Thanks

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"....Here's where problems started. For some reason we decided to try to see if the engine would turn before we tightened the timing chain."

Yep, ....... never, ever, try to turn the engine over without the timing chain tensioner in place.  What happens is the chain will bunch up on the small sprocket on the crankshaft.  Kicking it that way can damage the chain and / or the small sprocket, or both.  

So, you need to inspect the sprocket.  Look for bent / chipped tooth.  Get a flywheel puller made for the 04 YZ250F, or find a shop willing to pull it for you, $15 to $20; puller is about $25 but you'd have to wait to receive it.  Once its off, pull the stator plate that the coils are mounted to.  Takes a Torx T20 bit.  Once the plate is removed, that exposes the chain and sprocket.  Cross your fingers and hope the teeth are not damaged.  Just for safety, I'd still replace the cam chain if the sprocket is not damaged. .... about $25.

Good luck. 

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