Mikuni BS26 26mm Constant Velocity (CV) carburetor primarily used in mid-sized engines ranging from 125cc to 250cc
, such as those found on Suzuki, Yamaha, and Hyosung motorcycles. Unlike direct-slide carburetors, it uses a diaphragm-controlled slide
to maintain an optimal air-fuel ratio across varying throttle positions. Mikuni Power Core Specifications
These verified factory specs provide the baseline for maintenance and tuning: Venturi Diameter : Typically 100–120 (varies by application) : 17.5–25 Needle & Seat : 2.5mm seat / 5.0mm needle stem Float Height : 24mm (Standard for many BS series units) Fuel Requirement : Minimum 87 Octane Petrol Adjustment & Tuning Procedures
Proper tuning requires the engine to be at operating temperature and the bike to be upright. Pilot (Air/Fuel) Screw Initial Setting : Start at 2 to 2.5 turns out from lightly bottomed. Fine Tuning
: With the engine running, turn the screw in 1/2 turn increments until the engine stumbles, then turn it the opposite way until it stumbles again. The ideal setting is the center point between these two stumbles. : Do not exceed 3.0 turns out
, as the spring tension becomes insufficient and the screw may vibrate out. Idle Speed
: Adjust the idle screw to achieve the manufacturer's target RPM (typically found in your specific vehicle's service manual). Choke (Enricher) System : Ensure there is 1/16" to 1/8" of slack
in the choke cable. Lack of slack can cause the plunger to stay partially open, leading to a rich mixture and poor idling. Common Troubleshooting BS Carburetors - Mikuni Power
Mikuni BS26 Carburetor: The Ultimate Maintenance & Tuning Manual
The Mikuni BS26 is a staple in the world of small-displacement motorcycles and ATVs. Known as a "constant velocity" (CV) carburetor, it is famous for its smooth power delivery and fuel efficiency. However, even the most reliable Japanese engineering requires precise tuning to stay "verified" and functional. mikuni bs26 carburetor manual verified
This guide serves as your comprehensive manual for the Mikuni BS26, covering everything from basic identification to advanced jetting. 1. Understanding the CV Design
Unlike "slide" carburetors where your throttle cable physically lifts the needle, the BS26 uses a vacuum-operated diaphragm.
The Benefit: It automatically compensates for changes in engine load and atmospheric pressure.
The Catch: If the rubber diaphragm has even a pinhole leak, your bike will bog down or refuse to rev. 2. Verified Specifications & Components
Before turning any screws, ensure you are working with the correct parts. A standard BS26 typically features: Venturi Size: 26mm
Main Jet: Standard sizes range from #110 to #130 depending on the OEM application. Pilot Jet: Usually a #15 or #22.5.
Float Height: Typically 17mm to 18.5mm (measured from the gasket surface to the top of the float without compressing the spring). 3. Step-by-Step Cleaning & Inspection
If your engine is hard to start or won't idle, the pilot circuit is likely clogged.
Remove the Bowl: Check for "varnish" or green gunk caused by old ethanol fuel.
Clear the Pilot Jet: This is the most common failure point. The orifice is tiny; use a dedicated carb cleaning wire or compressed air. Never drill it out. Mikuni BS26 26mm Constant Velocity (CV) carburetor primarily
Inspect the Diaphragm: Hold the rubber slide diaphragm up to a bright light. If you see any light through the rubber, it must be replaced.
Check the Needle Valve: Ensure the rubber tip of the float needle is supple and hasn't developed a "ring" or groove, which causes overflowing. 4. Tuning & Adjustments (The Verified Way) The Pilot Screw (Idle Mixture)
On the Mikuni BS26, the pilot screw is usually located on the bottom (engine side). Turning IN (Clockwise): Leans the mixture (less fuel).
Turning OUT (Counter-clockwise): Richens the mixture (more fuel).
Standard Setting: Start at 2.0 to 2.5 turns out from lightly seated. The Needle Clip
If you experience a "flat spot" during mid-range acceleration (1/4 to 3/4 throttle), you may need to adjust the jet needle.
Lowering the clip (moving it toward the pointy end) raises the needle, making the mid-range richer.
Raising the clip (moving it away from the point) lowers the needle, making the mid-range leaner. 5. Troubleshooting Common Issues
Fuel Leaking from Overflow: Stuck float, dirty needle seat, or incorrect float height.
Backfiring on Deceleration: This usually indicates a "lean" condition. Try turning the pilot screw out 1/2 turn. Step 3: The Pilot Air Screw (PAS) Every
Engine "Hanging" Idle: This is often caused by an air leak. Spray starter fluid around the intake boot; if the RPMs jump, you have a leak. 6. Pro-Tip: The "Vacuum Test"
Once the carburetor is reassembled, manually push the slide up with your finger and release it. It should fall with a distinct "thump" and a slight delay. If it falls instantly like a rock, the diaphragm isn't sealed correctly, and the carb won't function under load.
Always remember: 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.
Every "verified" manual warns about this: Do not force the PAS. The BS26 uses a fine-tip screw with a tiny O-ring and washer.
The BS26 is a constant-velocity carburetor, meaning the slide position is not mechanically linked to the throttle grip. Instead, a vacuum port downstream of the throttle valve lifts the diaphragm slide against spring pressure.
Sequence of operation (per Mikuni engineering notes):
⚠️ Critical verification: The CV design intentionally delays slide rise, preventing bogging under sudden throttle openings.
The common mistake: measuring with bowl gasket compressed.
Correct method (per Mikuni manual):
Specs re-verified:
Symptoms of wrong height: