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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 could be wrong?

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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 could be wrong?

Broken woodruff key for the stator.

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 The guy that gave it to me sunk it in a swamp.

 

 

If it injested water while the engine was running there's a chance it hydrolocked, if so the connecting rod might be bent.

Edited by mlatour

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I have swapped the carb out with my wr250f's that i rebuilt last year. The Manuel says they are the exact same carb. After I did this it kept on backfiring, nothing changed.

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When I rebuilt the top end the cylinder was filled with swampy water and the bike wood not kick over. After it was rebuilt the bike was able to kick over and have lots of compression. Could hydrolock and I bent connecting rod still be the issue?

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I've never had an engine hydrolock, but since it was a known issue that it got swamped, wouldn't kick over & was full of water at disassembly

if I were you I would have a closer look at the rod's straightness and end play before reassembling it.

 

You mentioned that you replaced the piston, was the old one physicaly damaged or just worn?

Was the piston stuck (rusted) in the bore or did it come out easily?

Edited by mlatour

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I rebuilt the top end because the bike wouldn't kick over due to the pressure from the mud and water in the cylinder. The rings were worn a little and the piston wasn't too bad but I changed it anyways. The piston moved easily after taking the valves off.

I was thinking the problem had something to do with a timing issue in the electrical computers. I was reading up on cdi units. One article said that even though the bike has a spark, the cdi unit can be no good due to a timing issue inside. Has anyone heard of this problem before?

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