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Timing Issues

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I have rebuilt a 2003 yz250f and it will not start just backfire. I think it is a timing issue or something wrong with the computers. The bike is timed correctly and has a strong spark. What are some common problems that can cause this?

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Sometimes one of the shims will cock up sideways, close up a valve, and cause it to not start or backfire.

Qu:  Did you replace the crank, or was a top end only?

Reason I'm asking is on the older bikes you have to look for worn small cam sprocket.

That will make the timing slightly off... not a huge amount, but it can cause it to jump time.

Qu: have you cleaned out the carb?

Sometimes if the rebuild takes a long time, or the carb set up over the summer and had a little gas in it, the pilot jet will get clogged up, and need cleaned out or replaced.  

Qu: How old is the gas in the tank?  It will go back in 30-40 days in a plastic tank.

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