13/07/2026
🚀 Project YBR 139cc – The Ultimate Build is Happening! 🚀
I’m finally pulling the trigger on a massive engine overhaul for my Yamaha YBR 125, and I want to hear what you guys think about this setup! We are turning this into a high-compression, fast-accelerating machine.
Here is the complete spec list for the build:
🔧 Engine & Breathing:
57.3mm Big Bore Kit (bumping the displacement to roughly 139cc)
Shaved Cylinder Head to increase the compression ratio
Engine Head Porting along with Larger Sized Valves
+1mm High-Lift Camshaft to maximize top-end power
Bearing Rocker Arms
PZ27 Carburetor (the perfect sweet spot for fueling a 140cc engine without bogging)
⚙️ Transmission & Drive:
Custom 5-Plate Clutch Setup (sourcing a complete, pre-modified clutch boss directly from China through our KM STAR supply chain to handle the extra torque)
14T Front / 43T Rear Sprockets (geared for aggressive, rapid acceleration to top speed)
428H 118-Link Golden O-Ring Chain
⚡ Electrical & Ignition:
150cc Heavy-Duty Loader Ignition Coil for a thicker, hotter spark under high pressure
Unrestricted Racing CDI to bypass the stock rev limiter and let the new cam breathe
1-Step Colder Spark Plug to safely handle the extra heat from the high-compression setup
What do you guys think of these modifications? Drop your thoughts, advice, or questions in the comments below! 👇