The BS25 relies on precision-drilled air passages that are often omitted from basic diagrams. These internal passages provide air for the pilot circuit and can become clogged with varnish from old fuel. The gaskets, especially the "upper gasket o-ring" (5YP-14959-00) and "lower gasket o-ring" (5YP-14564-00), are critical for sealing internal vacuum passages. A patched diagram will note their locations, as leaks here will cause erratic running.
Dirt is trapped in the float needle valve, or the float height is incorrect.
Ensure that when installing a new mixture screw (or cleaning the old one), this tiny O-ring, along with the washer and spring, is installed in the correct order: Screw -> Spring -> Washer -> O-ring . 3. Step-by-Step Rebuild Guide (Following the Diagram) mikuni bs25 carburetor diagram patched
To help tailor this information to your specific project, tell me a bit more about what you are working on:
For a detailed, printable schematic, many enthusiasts utilize the Yamaha OEM Parts Finder at YamahaPartsHouse.com to find the precise BS25 layout for their specific bike model. 2. Common "Patched" Issues: Where the Diagram Meets Reality The BS25 relies on precision-drilled air passages that
Carefully seat the pilot air screw until it bottoms out gently, then back it out to the baseline specification noted on your patch notes (typically 1.5 to 2.5 turns out).
The output side that clamps into the rubber intake boot leading to the engine cylinder. A patched diagram will note their locations, as
Authentic Mikuni carburetors feature a "Mikuni Corp" casting on the slide and a "Made in Japan" mark on the body. Counterfeit "knockoffs," such as the Ucal BS29, often lack these markings.
To help find the exact specifications or parts for your rebuild, could you tell me:
: For specific tuning curves and effectiveness of parts (like the pilot jet vs. main jet ranges), the Mikuni Power Resources page offers functional charts that explain how adjustments impact performance. Key Specifications for BS25 Measurement Intake ID Intake OD Air Filter ID Air Filter OD Mikuni Carburetor Catalog