Keefer Tested

Top 6 Mods To The Yamaha YZ125

Comments (9)
  1. Great advice. Plus I feel the 2-stroke improves the riders skills & a stock bike makes them have to work more on their skills/mindset. I personally prefer 14/51 gearing. In my opinion the 14 tooth sprocket is bigger around which I feel raises the chain higher off the swing arm & improves handling.

  2. Have you considered adding a big bore kit, I believe is 144 cc? I have considered this but feel it would be best to have the crank and new larger piston assembly rebalanced. I know car engines that are perfectly balanced produce more horsepower because less horse power is lost through vibration. Vibration is energy that gets transferred into the engine block, Not the crank shaft drive gear.Do you think the big bore kit would make it necessary to make any changes to any of your other Power add-ons? The larger displacement would obviously improve the bottom end so I’m thinking the FMF pipe over Pro to better the top end.

    1. Did you consider the 144 cc big bore kit? I suspect the larger piston will be heavier. If so It would be best if the crankshaft, rod and piston assembly were balanced as a new unit. Vibration is energy that gets transferred into the entire motor and crank case extracting power from the crankshaft.

  3. I have a 16 year old Supermini rider that is looking to move up this season and beyond to a YZ125. He is a top 10 local kid but not a high revving rider since he does not have to be., He is 5’7″ and 95 lbs. What pipe/silencer combo would you recommend for his riding style? Our first order of business will be to get the suspension dialed for his riding style and weight then look at your 6 mod recommendations.

    Thanks for the help.

    1. Hi!!
      The first thing to do are to change the suspension spring

      Stock. Are for 135 to 155

      At 155 lbs the sag at 100 mm
      The free sag are at 30

      Motocross action tell the valving are awesome close to factory

  4. Bill H
    Thé YZ have a phenomenal stock suspension
    Maybe just change the spring because stock are for 135-155 lbs

    At 155 lbs sag at 100 mm the free sag are at 30 mm

    Free sag 25 to 40

  5. Excellent points Kris Keefer. Thank you for putting this together. Just purchased a used YZ125 as a transition for my son. Looking forward to taking these steps.

  6. I’m 120lbs running stock suspension (for now) what changes and sag can I make to make it the best possible?

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