The Mikuni BS25 is a Constant Velocity (CV) round-slide carburetor commonly used in 150cc 4-stroke engines, such as the GY6 platform. A "patched" diagram typically refers to a modified or annotated exploded view that includes updated part numbers, tuning notes, or corrections from the original factory manual to assist with modern rebuilds. Key Specifications of the Mikuni BS25 Bore Size: 25mm inner diameter on the intake side.
- You will miss the pilot air screw. Without the patch, you’ll reassemble the carb with the brass plug still in place, set the mixture screw to “2.5 turns” based on guesswork, and never achieve a steady idle.
- You will install the float upside down. The original diagram shows the float as a symmetrical shape. It isn't. The patched version shows the mold mark that faces DOWN.
- You will misplace the small spring for the diaphragm vent. This spring sits in the top cap. The patched diagram circles it because it falls out during disassembly and rolls under the workbench.
- Remove the top cover (4 screws). The diagram patch shows a warning: "Do not lose the spring under the diaphragm."
- The diaphragm should be pliable. The patched diagram has a "crack check" illustration.
- The piston valve (slide) – The patch highlights the two grooves that allow air to bypass at idle. Clean these with contact cleaner only.
Why a “Patched” Diagram Matters
Original factory diagrams often omit:
Flooding / overflowing bowl
Reassembly Notes from the Patched Diagram
When rebuilding a BS25:
Mikuni Bs25 Carburetor Diagram Patched
The Mikuni BS25 is a Constant Velocity (CV) round-slide carburetor commonly used in 150cc 4-stroke engines, such as the GY6 platform. A "patched" diagram typically refers to a modified or annotated exploded view that includes updated part numbers, tuning notes, or corrections from the original factory manual to assist with modern rebuilds. Key Specifications of the Mikuni BS25 Bore Size: 25mm inner diameter on the intake side.
- You will miss the pilot air screw. Without the patch, you’ll reassemble the carb with the brass plug still in place, set the mixture screw to “2.5 turns” based on guesswork, and never achieve a steady idle.
- You will install the float upside down. The original diagram shows the float as a symmetrical shape. It isn't. The patched version shows the mold mark that faces DOWN.
- You will misplace the small spring for the diaphragm vent. This spring sits in the top cap. The patched diagram circles it because it falls out during disassembly and rolls under the workbench.
- Remove the top cover (4 screws). The diagram patch shows a warning: "Do not lose the spring under the diaphragm."
- The diaphragm should be pliable. The patched diagram has a "crack check" illustration.
- The piston valve (slide) – The patch highlights the two grooves that allow air to bypass at idle. Clean these with contact cleaner only.
Why a “Patched” Diagram Matters
Original factory diagrams often omit:
Flooding / overflowing bowl
Reassembly Notes from the Patched Diagram
When rebuilding a BS25:
