Yamaha YZ125 Racing Engine Mods
Unlock the full potential of your YZ125. We correct the factory short side radius and optimize the combustion chamber to deliver 12,000+ RPM of usable power.
The Stock Bottleneck
"Conservative factory port timing that signs off early compared to Austrian rivals."
The HP Race Dev Solution
"Aggressive transfer port hook modifications to extend the RPM ceiling."
We don't just "grind ports." We mathematically correct the engine's geometry to solve this specific flaw.
Engineering the YZ125 Platform
The Yamaha YZ125 suffers from a 'short side radius' in the exhaust port, where the airflow effectively separates from the port wall at high RPM, creating turbulence that chokes top-end power. We reshape this radius to maintain laminar flow, allowing the engine to pull hard well past the factory sign-off point.
For 2022+ models, the new Keihin PWK38S carburetor is a huge improvement, but the intake boot/reed cage junction is still flawed. We match this junction to ensure the increased airflow from the carburetor actually reaches the crankcase without disruption.
We set up the combustion chamber volume based on your fuel availability. For pump gas riders, we slightly retard timing and increase dome volume to prevent detonation. For race fuel (C12/T2), we tighten the head significantly to leverage the higher burn rate for maximum torque.
Select Your Level
Choose the package that fits your racing class.
YZ125 Platform
125cc PlatformMinimum Spec
Reliability, clean-up, throttle response. The entry-level fix.
- •Cylinder Truing & Casting Cleanup
- •Power Valve Mechanism Check & Calibration
- •Deck Height Verification
Recommended For
Local Racing & Practice Bikes
Privateer Spec
The sweet spot. High performance, durable enough for a full season.
- •Stage 2 Cylinder Porting
- •Cylinder Head Machining (Squish & Combustion Chamber)
- •Exhaust Port Polishing
- •Carburetor Taper Bore
Recommended For
Regional Championships & Competitive Stock Class
Factory Works Spec
No compromise. Maximum power, exotic coatings, extensive labor.
- •Stage 3 Full Race Porting
- •Engine Case Matching (Blueprinting)
- •Ceramic Coatings (Piston Crown & Combustion Chamber)
- •ISF Process Transmission Tumbling
- •Dynamically Balanced Crankshaft
Recommended For
Loretta Lynn's, Mini Os, Pro Nationals
Common Questions: YZ125 Platform
Do I need to switch my pre-2022 YZ125 to a Keihin carb?
It is highly recommended. The stock Mikuni TMX is notoriously difficult to jet. We offer a Keihin conversion kit that pairs perfectly with our porting for stable, crisp fueling.
Does the 2022+ model need porting?
Absolutely. While the 2022 update improved the intake tract, the cylinder casting still has significant flaws that disrupt airflow at high RPM. Our 'Factory Spec' cleans this up to unlock the engine's true potential.