Question about 2003 Yamaha YZ 125

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Yz 125 99 won't run

New top end, new plug, carb cleaned, good spark, grey gearbox oil so changed crank seals, fresh fuel getting to plug, it won't fire HELP PLEASE

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  • pearhead Jun 17, 2009

    Do you mean the crank seal fitted properly?

  • pearhead Jun 22, 2009

    My chassis no is JYACE10C53AOO8555 can you confirm this is a 2003 machine Thanks

  • Anonymous Mar 30, 2014

    Cranks good, good fuel pressure .

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You need to ensure for the Carburettor Needle fixed, so that the fuel injected out properly, also if the Crank Gasket should be fitted properly.

Posted on Jun 17, 2009

  • Puneet Jain
    Puneet Jain Jun 17, 2009

    Yes need to check this also

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1 Answer

1989 yamaha banshee no start


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The only way that can happen is if gas is overflowing in the carb and running down the cylinder wall and into the exhaust port and / or into the crankcase. Gearbox oil is separate from the crankcase. The crank gets lubrication from the gas/oil mix. Be sure to turn off the gas valve at the tank when the bike is not in use. If you can manage it, pull the head off, (or at least remove the spark plug ), then, with the piston at bottom of the stroke, turn the bike upside down to drain the gas from the crank. I would then remove and clean the carb to be sure the float is not sticking open. Be sure the overflow line is not blocked. Install an in-line fuel filter. Please rate my answer. Thanks.

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1 Answer

Bought a 2002 Rm125 about 2 months ago, with a fresh top end....First ride was great with impressive power throughout. About two weeks later I noticed that it was missing some about half throttle , and...


My first thought is low compression... which leads to a few questions...
What fuel to oil ratio have you used?
What do you mean by 'fresh top end'? To me a fresh top end includes piston and rings and cylinder bore, gaskets and crank seals. Anything less on a 2-stroke is an incomplete job and usually leads to premature failure.
Have you followed break-in procedure on your fresh top end? Double check the head bolt torque.

Do a compression check first then go from there.

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yeah something you wanna check next, compression. if you can get your hands on a compression tester.

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Your bike is water cooled. Some of that water is leaking into the gear box. The cylinder gasket is the likely leak. I would let the shop take a look at it instead of trying to fix it yourself.

But be sure there is not an external source for the water. Do you wash the bike often? Ride the creek bed often? Leave the bike where the lawn sprinkler gets the bike wet? Leave it out in the rain? Change the oil and keep the bike dry. If the problem comes back then go see the dealer.

Mar 19, 2009 | 2003 Yamaha YZ 125

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