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.
The common mistake: measuring with bowl gasket compressed.
Correct method (per Mikuni manual):
Specs re-verified:
Symptoms of wrong height:
Verified Sources
The information provided in this article has been verified through the following sources:
Conclusion
The Mikuni BS26 carburetor is a reliable and performance-oriented carburetor model. By following the verified guide provided in this article, you can ensure proper adjustment, maintenance, and repair of your Mikuni BS26 carburetor. Remember to consult the official Mikuni carburetor manual and other verified sources for specific information on your carburetor model.
Additional Resources
For more information on the Mikuni BS26 carburetor, you can consult the following resources:
Specifications
Disclaimer
The information provided in this article is for general information purposes only. Consult the official Mikuni carburetor manual and other verified sources for specific information on your carburetor model. The author and publisher disclaim any liability for damages or injuries resulting from the use of the information provided in this article.
Comprehensive Mikuni BS26 Carburetor Verified Manual and Tuning Guide
The Mikuni BS26 is a Constant Velocity (CV) carburetor widely used in small-to-medium displacement motorcycles and ATVs, such as the Suzuki GN250 and Hyosung GT250. Unlike mechanical slide carburetors, CV carburetors use engine vacuum to lift the throttle slide, providing smoother power delivery and better fuel efficiency.
This verified guide covers the operation, maintenance, and tuning specifications for the BS26 series. 1. Core Operating Systems
The Mikuni BS26 functions through five overlapping metering systems that control the fuel-air mixture across different RPM ranges:
Pilot Circuit: Controls idle and low-speed operation (up to 1/4 throttle). Throttle Valve: Regulates air intake and vacuum levels. mikuni bs26 carburetor manual verified
Needle Jet & Jet Needle: Manages the mid-range (1/4 to 3/4 throttle).
Main Jet: Governs high-speed and wide-open throttle (WOT) performance.
Starter (Choke) Circuit: Provides a rich mixture for cold starts. 2. Technical Specifications
While exact settings can vary by vehicle model (e.g., the Hyosung GV250 uses a "BDS26" variant), standard verified specs for the BS26 include: Standard Specification Bore Size Idle RPM 1,400 ± 100 RPM (Typical) Float Height ~17mm (varies by application) Pilot Screw 1.5 to 2.0 turns out from seated 3. Maintenance & Cleaning Procedure
To restore performance, a thorough teardown is often more effective than a generic "rebuild kit" if internal parts are not physically damaged.
Disassembly: Remove the float bowl and top vacuum cap. Be careful not to tear the rubber diaphragm attached to the slide.
Jet Removal: Carefully unscrew the Main Jet and the recessed Pilot Jet. Use a precisely fitting flathead screwdriver to avoid stripping the soft brass.
Cleaning: Use specialized carburetor cleaner and compressed air to clear internal passageways. Ensure the tiny orifices in the pilot jet and the holes in the emulsion tube are completely clear.
Inspection: Check the needle valve (float needle) for a "ring" or wear mark on its tip, which can cause flooding. 4. Tuning and Adjustment Idle Circuit (Air/Fuel Screw) Adjust this circuit with a warmed-up engine: The Mikuni BS26 is a constant-velocity (CV) carburetor
Turn the pilot screw in/out in 1/2 turn increments every 10 seconds until the engine reaches its highest, steadiest idle.
Rich/Lean Indicators: If you must turn the screw more than 3 turns out, the pilot jet is likely too small. If bottoming the screw doesn't kill the engine, the jet is too large. Mid-Range (Jet Needle)
The jet needle features a clip that can be moved into different notches: Air screw adjustment Mikuni carbs only
With BS26 on parallel twins (e.g., GS500), syncing is mandatory:
Before reinstalling on the bike:
If you are rebuilding the carburetor, familiarize yourself with the parts layout:
Before you turn a single screw, you need to understand what you are working with. The "BS" stands for Bore Size Symmetrical, and "26" refers to the 26mm venturi diameter.
Unlike a flat-slide racing carburetor, the BS26 is a Constant Velocity (CV) unit. This means the throttle plate (butterfly valve) is directly connected to your throttle cable, but the slide (piston valve) is controlled by engine vacuum and a diaphragm.
Why this matters for your manual search: Specs re-verified :