The Mikuni BS26 is a 26mm Constant Velocity carburetor utilizing a vacuum-operated diaphragm, commonly tuned with a pilot screw set 1.5 to 2 turns out. Key maintenance involves cleaning the jet circuits, checking for varnished fuel, and ensuring the float needle seats properly. For detailed schematics, view the Mikuni BS26 Manual on Scribd
Before reinstalling on the bike:
Flip the carburetor over. Remove the four screws securing the bowl. If stuck, tap the side gently with a plastic mallet.
The Mikuni BS26 is not just a collection of jets and passageways; it is a precisely engineered vacuum-operated constant-depression (CV) carburetor often found on popular 125cc machines, such as the Suzuki Intruder 125, Suzuki VanVan 125, and the Aprilia Leonardo 125. Understanding its operation is the first step to unlocking your engine's full potential. mikuni bs26 carburetor manual verified
Because this carburetor was manufactured under contract by Hitachi (to Mikuni specifications) primarily for Suzuki, standalone "Mikuni" manuals are rare. The "verified" data is found in the official .
| | Details / Values | Notes | | :--- | :--- | :--- | | Type | Constant Depression (CV), Diaphragm-type, Vacuum-operated | Features a vacuum piston (skep) with a rubber diaphragm. | | Venturi Diameter | 22.2 mm (oval) | Commonly found in BS26-49, BS26-61 and similar variants. | | Identification Number | Stamped on the carburetor body (e.g., BS26-49, BS26-61, BS26SS). | The specific model number identifies its original factory settings. | | Main Jet | Variable; often around #107.5 (varies by model) | Model-specific; e.g., BS26-49 specs list a 107.5 main jet. | | Pilot Jet | Variable; often around #42.5 (varies by model) | Works with the pilot air screw to control idle mixture. | | Float Height | 21.4 mm ± 1 mm | This is the correct specification for the BS26SS variant. | | Idle Speed | 1600 ± 100 RPM | Set after the engine is at full operating temperature. | | Application | Aprilia Leonardo 125 (BS26-49, BS26-61), Suzuki GS-R125, Suzuki RV125 VanVan (2003-06), Suzuki Intruder 125 | Also found on Suzuki AN125/150 Burgman and the Yamaha Byson 125. |
, such as those found on Suzuki, Yamaha, and Hyosung motorcycles. Unlike direct-slide carburetors, it uses a diaphragm-controlled slide The Mikuni BS26 is a 26mm Constant Velocity
| Parameter | Spec | |-----------|------| | | Constant Velocity (CV), downdraft | | Bore size | 26 mm | | Venturi diameter | 24.5 mm (effective) | | Fuel inlet | 5/16" (8mm) barb | | Float bowl gasket | Rubber (O-ring type) | | Pilot air jet | 120–130 (varies by model) | | Main air jet | 180–200 | | Idle speed | 1,200–1,300 RPM |
Ensure the diaphragm seats correctly in the rim of the carburetor body before closing the top cover. 4. Common Troubleshooting Potential Cause Hard to Start (Cold) Choke not functioning / Clogged Pilot Jet Clean pilot jet; check starter plunger cable. Idle Hanging / High Vacuum leak / Pilot screw too lean Check intake boot; turn pilot screw out. Bogging at Mid-range Jet needle clip position wrong Move needle clip up/down. Hesitation at WOT Main jet clogged / Too small Clean or replace main jet. Fuel Leaking from Overflow Float needle sticking / Float level set wrong Clean needle valve; adjust float height. 5. Summary of Verified Tips Air Screw: Affects only idle/low-speed. Needle: Affects midrange fueling. Vacuum Leak: Often causes high, erratic idle.
Carefully lift the cover, noting that the diaphragm spring will push upward. Remove the four screws securing the bowl
The pilot screw regulates the idle fuel circuit. It is located on the engine side of the carburetor body.
Here are some common problems and troubleshooting tips for the Mikuni BS26 carburetor:
Issue 3: Hanging Idle (Engine Stays at High RPM After Letting Go of Throttle)
When working on a Mikuni BS26, cleanliness is more important than mechanical strength. Use a high-quality carb cleaner and ensure every tiny passage is verified clear before reinstallation.
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