Question about 2003 Yamaha YZ 125

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I have recently done a engine rebuild and my engine wont start i have compression and a spark and i have set the timing up accoring to the manual, and ideas to what may be wrong? thank you .

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Did you put the reeds back in correctly? Is the spark plug getting wet from fuel when you try to start it? Does it start with Starting fluid? Will it pull start (or push start?) Could be lots of things. Tell me what you've tried. I need more info to narrow it down.

Posted on Nov 22, 2010

  • 4 more comments 
  • ryan-dundas Nov 23, 2010

    i think its not getting enough compression the piston ring is half ceased onto the piston and a mechanic i spoke to said he would advice a new piston. thank you for your help

  • Eric Cross
    Eric Cross Nov 24, 2010

    I advise Pro-X pistons. Your welcome. Was the ring seized on the exhaust side of the piston? (nearest the arrow)

  • ryan-dundas Nov 25, 2010

    from riding position just nearest the left side of the exhaust port, why?

  • Eric Cross
    Eric Cross Nov 28, 2010

    Sorry took awhile, Thanksgiving and all. . .Ok, the ring seizes on the exhaust side due to a large temperature gradient from the intake to the exhaust side. The intake side is cool from incoming charge and the exhaust side is hot from outgoing gases. A large temp difference occurs following engine rebuilds, exhaust mods, or intake filter mods when a lean condition is created. Lean is when your air to fuel ratio does not have enough fuel. You can add fuel by LOWERING the C-clip on the main jet needle. This raises the needle position, giving you a more fuel to air. Look at your spark plug as-is, white and ashy???= Lean. Dark and carbon fouled???=Rich. You want a slightly rich mix. Jet kits are inexpensive and I advise to buy 2 sizes larger and 2 sizes smaller than stock so that you can adjust the mix throughout the year. Outside temp changes (especially cold) can cause your air/fuel ratio to change to lean. Lean is very bad for engines, resulting in seized rings, locked up pistons and melted spark plug tips. Also be sure to align the ring correctly. All 2 stroke engine pistons have locator pins on the rings. The opening on the ring goes here, the stamped letters on the ring face up.

  • ryan-dundas Nov 29, 2010

    ok thank you very much, really helped me .

  • Eric Cross
    Eric Cross Nov 29, 2010

    No Problem. Need anything else?



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