Turn the screw in or out in 1/4-turn increments until you find the highest, smoothest idle speed.
Q: How do I adjust the idle mixture on my Mikuni BS26 carburetor? A: Turn the pilot screw counterclockwise to richen the mixture or clockwise to lean the mixture.
Ensure there are no tears or pinholes in the black rubber diaphragm. A tear prevents the slide from lifting.
Tuning must follow a specific sequence, from the bottom of the RPM range to the top.
Unfortunately, Mikuni doesn't freely publish a dedicated BS26 manual online. Your most reliable sources are the for the specific bikes that used this carburetor. For the BS26, some of the best official documentation exists within: mikuni bs26 carburetor manual verified
The BS26 operates using five primary metering systems that regulate the air-fuel mixture between 1,750 and 4,500 RPM:
The best manuals are the original equipment manufacturer (OEM) publications. For the most part, a dedicated "Mikuni BS26 manual" does not exist as a single publication. Instead, the data is contained within the specific motorcycle's factory service manual.
The Mikuni BS26 carburetor is a vacuum-controlled, slide-type carburetor designed for use on motorcycles, ATVs, and small engines. It features a 26mm throttle bore and is capable of delivering a wide range of fuel flow rates to suit various engine applications.
The Mikuni BS26 carburetor features a range of jetting options to suit various engine applications. The following jetting guide provides a general starting point for tuning: Turn the screw in or out in 1/4-turn
Turn the pilot screw inward 1/4 turn at a time. Listen for the engine RPM to drop or stumble. Stop, and turn the screw outward 1/4 turn at a time past the baseline until the RPM peaks. Set the screw halfway between the rich drop-off point and the lean drop-off point. Adjust the idle speed screw back to standard if the idle speed changed during this process.
Unlike "slide" carburetors where your throttle cable physically lifts the needle, the BS26 uses a vacuum-operated diaphragm.
The is a 26mm CV (Constant Velocity) carburetor found on many small-displacement, single-cylinder motorcycles and ATVs, most notably the Suzuki GN250 and GZ250 Marauder . Known for its reliability, it relies on vacuum pressure to lift the slide rather than a direct cable connection.
| | 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. | Ensure there are no tears or pinholes in
The Mikuni BS26 is a constant velocity (CV) carburetor widely used in small displacement motorcycles like the Suzuki GN125 and Yamaha XV250. Achieving peak performance requires adhering to verified manual specifications for maintenance and tuning. Understanding the Mikuni BS26 System
Located at the top of the carburetor. It reacts to engine vacuum.
Turn the pilot screw clockwise until it gently seats. Do not overtighten, as this will damage the brass tip and the aluminum seat. Back the screw out as a baseline starting point. Warm the engine to operating temperature.