| Symptom | Unpatched Diagram Diagnosis | Patched Diagram Diagnosis | | :--- | :--- | :--- | | Flooding gas | Replace float needle | Replace the hidden o-ring behind the seat (patch #2) | | No idle | Clean pilot jet | Clean the horizontal passage between pilot jet and mixture screw (patch #1) | | Bogs at half-throttle | Replace diaphragm | Check diaphragm tab alignment and spring placement (patch #4) | | Mixture screw does nothing | Unscrew further | Drill out the brass anti-tamper plug (patch #3) |
The Mikuni BS25 is a constant vacuum (CV) carburetor commonly found on small-to-medium displacement motorcycles (e.g., Suzuki GN250, Yamaha SR250, early Kawasaki Z250). In workshop parlance, a “patched diagram” refers not to a software patch, but to a manually corrected, annotated, or repaired schematic—often used to overcome design flaws, wear, or tuning issues.
This report analyzes a “patched” version of the BS25 diagram, focusing on common failure points, aftermarket modifications, and how the patch alters original Mikuni engineering. mikuni bs25 carburetor diagram patched
The BS25 has a small barbed fitting for the fuel petcock (auto fuel valve).
The original diagram shows the pilot jet screwing into a blind hole. This is wrong. The Patch: A red circle or arrow connecting the pilot jet cavity to the three small bleed holes downstream of the throttle plate. The patched diagram highlights that fuel must travel horizontally through a tiny drilling that you cannot see without removing the mixture screw. | Symptom | Unpatched Diagram Diagnosis | Patched
Patching a diagram doesn’t guarantee success. Field reports note:
If you are looking at a diagram because you need to repair a patched carb, stop patching. Follow this flowchart of salvation: Are the plastic vacuum nipples broken
If using a patched BS25 diagram:
The term “patched” on a diagram indicates one of three real-world interventions:
| Patch Type | Diagram Marking | Purpose | |------------|----------------|---------| | Jet modification | Red circle around pilot jet | Replacement with larger jet (e.g., #42.5 → #45) | | Drilled passage | Dashed line added | Enlarging air bleed hole for richer idle | | Gasket repair | Cross-hatched area | Silicone or paper gasket patch for vacuum leak |
