Manual Verified: Mikuni Bs26 Carburetor
The Mikuni BS26 is a constant-velocity (CV) carburetor widely used across small-to-mid-displacement Japanese motorcycles from the late 1980s to the early 2000s, notably the Suzuki GS500, GN250, and early Kawasaki Ninja 250. Unlike direct-slide carburetors, the BS26 uses intake vacuum to control the slide, offering smooth throttle response and consistent fuel metering despite altitude or air filter changes. This essay synthesizes data from official service manuals—particularly the Suzuki GS500 Supplementary Service Manual (99500-36040-03E) and Mikuni BS26 Tuning Guide—to detail its operation, adjustments, and common failure modes.
Searching for "Mikuni BS26 carburetor manual verified" often returns generic Mikuni tuning books. Those are helpful, but they lack model-specific emissions data.
This is the heart of your search for a "Mikuni BS26 carburetor manual verified." Below are the factory-fresh specifications from Suzuki literature. Note that emissions regulations changed by year, so we have listed the most common (1994-2000 GS500 baseline, which applies to most BS26 units). mikuni bs26 carburetor manual verified
| Specification | Verified Value | Notes | | :--- | :--- | :--- | | Main Jet | #127.5 (Left) / #130 (Right) | GS500 only. Singles use #125. | | Pilot Jet | #40 | (Some models use #37.5 – check your needle) | | Pilot Air Screw | 2.5 to 3.0 turns out | From fully seated. Do not exceed 4 turns. | | Float Height | 13.0 mm (13.7 mm for GN250) | Measure with carb inverted, float tang just touching needle spring. | | Needle Clip Position | 4th groove from top | (Standard is middle, but GS500 uses richer 4th) | | Idle Speed | 1,200 RPM (+/- 100) | Must be set when engine is fully hot. | | Diaphragm Part # | 13258-02C00 | (OEM Mikuni – Aftermarket fails quickly) | | Float Bowl Gasket | 13269-01A00 | Not reusable after soaking in cleaner. |
Per the Suzuki GS500 Service Manual, section 4-7: The Mikuni BS26 is a constant-velocity (CV) carburetor
Float height check:
Diaphragm inspection:
Jet identification: | Circuit | Typical BS26 range | GS500 (1996–2000) | |---------|-------------------|--------------------| | Pilot jet | 35 – 45 | 37.5 | | Main jet | 120 – 140 | 125 | | Pilot air jet | 80 – 120 | 95 | | Needle clip position | Groove 1–5 | 3rd from top |
The BS26 uses an air screw on the inlet side of the carburetor body. Turning it out leans the idle mixture; turning it in richens it. Diaphragm inspection:
Factory baseline (GS500 manual): 2.5 turns out from lightly seated. Fine-tuning method:
| Symptom | Probable cause | Manual verification step | |---------|----------------|---------------------------| | Hesitation at 5000 rpm | Torn diaphragm | Visual inspection; blow lightly into vacuum port—slide must rise smoothly | | Hard start cold | Clogged starter circuit | Remove starter plunger; check tiny fuel passages with carb cleaner | | Rich idle (black plugs) | Pilot screw too far in | Reset to 2.5 turns out; replace pilot jet if >3.5 turns required | | Surge at steady throttle | Incorrect float height | Remeasure at 30° angle; adjust tang gently | | High idle (1800+ rpm) | Vacuum leak or stuck slide | Spray propane around intake boots; check slide for free movement |
