Question about Yamaha YZ 125 Motorcycles

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HI, I HAVE A 1991 YAMAHA YZ125 2-STROKE. I HAD THE CYLINDER BORED OUT AND I PUT A SLIGHTLY LARGER PISTON IN. THE BIKE STARTS FINE BUT WON'T STAY RUNNING AS IF NOT GETTING ENOUGH GAS. ANY IDEAS? THANKS, TOM.

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  • Tom Allert May 01, 2010

    HI, BEFORE I PUT THE NEW PISTON IN THE BIKE RAN FINE, I ALSO PUT NEW REEDS IN. COULD IT BE A CHOKE CABLE OR A CARB ADJUSTMENT? THE BIKE DOESN'T STAY RUNNING LONG ENOUGH TO DRIVE IT ANYWHERE. THANKS,TOM.

  • Tom Allert May 01, 2010

    AFTER HAVING THE CYLINDER BORED OUT AND A NEW PISTON INSTALLED THE BIKE STARTS FINE BUT DIES OUT. IT'S A 1991 YAMAHA YZ125 2-STROKE. DOESN'T STAY RUNNING LONG ENOUGH TO DRIVE IT.

  • Tom Allert May 01, 2010

    I AM RUNNING HIGHER OCTANE, COULD IT BE A CHOKE CABLE OR CARB ADJUSTMENT?

  • Tom Allert May 01, 2010

    THANKS ALOT, THE INFO WILL HELP. TOM.

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You are correct... bigger piston equals larger displacement and therefore more gas!!!

Did you shave the head to? if so you now have higher compression and will need to run higher octane fuel

good luck

Posted on May 01, 2010

  • Sean Reisdorf
    Sean Reisdorf May 01, 2010

    you do NOT want higher octane unless you shaved the heads to make it a higher compression engine

    if all you have is big displacement then you need to increase the fuel mixture screws til the bike starts running



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Hi Tom. I recommend that you go to an auto parts store and get a bottle of a fuel additive called Seafoam. Follow the directions on the bottle regarding dosage and run it through your engine. Within a couple of miles you will be amazed. IF you can not actually run the engine enough to go for a ride, get a can of starter fluid and spray enough into the air intake to keep the engine running for a few minutes. Both products will gradually solve your problem. Joe

Posted on May 01, 2010

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http://www.bikez.com/motorcycles/yamaha_xt_125_r_2006.php
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http://cgi.ebay.com/ebaymotors/Yamaha-Blaster-040-Platinum-Piston-Engine-Rebuild-Kit-_W0QQcmdZViewItemQQhashZitem1c1ad757dbQQitemZ120709404635QQptZMotorsQ5fATVQ5fPartsQ5fAccessories

The crankshaft is also lubricated by the oil in the fuel. There is a possibility that the crankshaft is also toast due to lack of lubrication. You will have to tear the engine down (top end) to verify the damage. you could remove the exhaust and look into the exhaust port to view the condition of the piston.

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Please rate this solution. Thanks garethholleb!

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While riding my yz 125 it just died and won,t kick up again


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  • While at the parts counter, get a new stock spark plug and a supply of Yamalube Two Stroke Motor Oil. You need to add 4 ounces per gallon when filling up. This will be a 32/1 gas to oil ratio. NEVER ride the bike without at least the needed 32/1 mix.
Please rate the solution as a "FixYa" if able. Thanks!

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