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Backfiring No Start

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I have a YZ250F 2003 that i am rebuilding. The guy that gave it to me sunk it in a swamp. I have;

 

  • changed piston and rings and has compression
  • checked valves and all are good
  • checked the rotor and stator for any damage and both are fine
  • cleaned carb and put back to factory settings (also swapped carbs just in case)
  • I did timing, so it is perfect
  • There is a strong spark

but the bike will only backfire and not start. I think it is the electrical but i dont know. what else could i do or what be wrong?

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Cam timing

 

Use the correct TDC mark on the right side inspection hole.

There are two marks, one is correct.

 

Count the number of timing chain pins with both cams facing out, and lined up right on the deck marks.....it should be 13 pins.

 

Hopefully you put in a new timing chain and tensioner, as they go bad fast if they are not submersed in oil all the time.

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I have done the timing to the correct TDC markings in the inspection hole. I have not put a new timing chain and tensioner in but everything lines up correctly. Could that still be the issue?

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I have had this issue before with quite a few 250s like krannie says timing needs to be perfect count the pins very closely, most of the time your issue is caused by the valves although they may be in spec they may not seat correctly causing your issues.

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