Because parts are nearly impossible to find, the manual must include a detailed exploded view with OEM Mikuni part numbers. Key components:
Before diving into the manual, let’s establish the hardware. The Mikuni BDS26 is a diaphragm-type (constant velocity) carburetor with a 26mm bore. Unlike traditional slide carburetors, the BDS series uses engine vacuum to lift the slide, offering smoother throttle response in variable load conditions like deep snow or uneven terrain.
You will most commonly find the BDS26 on:
Because these carburetors are no longer in production, a service manual is your lifeline for keeping your machine running.
Post-Service Checklist
Additional Tips and Precautions
This guide provides a general overview of the Mikuni BDS26 carburetor service manual. Always consult the manufacturer's instructions and take necessary precautions to ensure a safe and successful service experience.
It dissolves the diaphragm rubber and the plastic vacuum piston guide rings.
If you strictly need the official PDF diagrams:
Disclaimer: This guide is for educational purposes. Carburetor tuning involves flammable liquids. Work in a well-ventilated area away from open flames.
Maintaining and tuning your Mikuni BDS26 carburetors is essential for keeping your motorcycle running smoothly. These constant velocity (CV) carburetors were a staple on many popular mid-sized bikes, most notably the Yamaha Virago XV250 and various Suzuki models. When they are clean and synchronized, they provide excellent fuel economy and crisp throttle response; when they aren't, you'll face idling issues, popping, and poor performance.
This guide serves as a comprehensive service manual to help you tear down, clean, and rebuild your Mikuni BDS26 assembly. Anatomy of the Mikuni BDS26
Before turning a wrench, it is important to understand the key components you will be handling: mikuni bds26 service manual
The Diaphragm and Slide: Located under the top cap. The rubber diaphragm uses vacuum to lift the slide and needle. The Float Bowl: The bottom chamber where fuel is stored.
The Float and Needle Valve: These regulate how much fuel enters the bowl.
Jets: The Pilot Jet (low speed/idle) and Main Jet (high speed/wide-open throttle). Pilot Screw: Adjusts the air-fuel mixture at idle. Tools and Supplies Needed
JIS Screwdrivers: Standard Phillips heads often strip Japanese carburetor screws. Use JIS bits if possible.
Carburetor Cleaner: A high-quality spray or a dip (like Berryman’s Chem-Dip).
Compressed Air: Essential for blowing out tiny internal passages.
Rebuild Kit: Including new O-rings, gaskets, and a float needle.
Ultrasonic Cleaner (Optional): The gold standard for deep cleaning. Step 1: Disassembly
Remove the Tops: Unscrew the top covers. Carefully pull out the slide and diaphragm. Inspect the rubber for pinholes or tears by holding it up to a light. If it’s torn, the slide won’t lift, and the bike will lose power.
Access the Bowls: Flip the carbs over and remove the four screws holding the float bowls. If they are stuck, a light tap with a plastic mallet can help.
Remove the Floats: Push out the hinge pin and lift the float and the attached needle valve.
Extract the Jets: Use a flathead screwdriver to remove the Main Jet and the Pilot Jet. The Pilot Jet is often recessed in a deep hole—ensure your screwdriver fits perfectly to avoid stripping the brass. Step 2: Deep Cleaning Because parts are nearly impossible to find, the
The BDS26 has very narrow passages that clog easily with old, "varnished" fuel.
Soak the Brass: Place the jets and needle in carb cleaner. Do not soak rubber parts or the plastic float.
Clear the Passages: Spray carb cleaner through every hole in the carb body. You should see fluid exiting from another hole. If not, it’s blocked.
Air Blast: Follow up with compressed air to dry the passages and dislodge any remaining debris.
Pilot Circuit: Pay special attention to the tiny holes near the throttle butterfly valve. These are critical for a steady idle. Step 3: Inspection and Float Height
Check the needle valve tip. If it has a visible ring or groove, it won’t seal properly, leading to a leaking carb.
Setting the Float Height:For the Mikuni BDS26, the float height is typically measured from the gasket surface (without the gasket) to the top of the float while the carb is tilted so the float tang just touches the needle pin.
Common Spec: Usually around 12mm to 14mm (check your specific bike's factory service manual for the exact millimeter).
Adjustment: Carefully bend the small metal tang on the float to reach the correct height. Step 4: Reassembly
Install Jets: Hand-tighten the Pilot and Main jets. Do not overtighten brass into the aluminum body.
Replace O-Rings: Always use new O-rings on the pilot screw and fuel joint pipes.
The Diaphragm: When reinstalling the slide, ensure the diaphragm tab is seated in its notch. If it’s pinched, the vacuum seal will fail. Because these carburetors are no longer in production,
Pilot Screw Initial Setting: Gently turn the pilot screw in until it seats, then back it out to the factory starting point (usually 2.0 to 2.5 turns). Step 5: Sync and Fine Tuning Once the carburetors are back on the bike:
Bench Sync: Before installing, ensure both throttle plates open at the exact same time by adjusting the linkage screw.
Vacuum Sync: Once the engine is warm, use a vacuum gauge or Manometer to perfectly balance the pull between the two cylinders.
Idle Adjustment: Use the idle speed screw to set the RPM to your manufacturer's recommendation (usually 1,100–1,300 RPM). Troubleshooting Common Issues Overflowing/Leaking: Stuck float or worn needle valve.
Backfiring on Deceleration: Clogged pilot jet or lean pilot screw setting.
Bogging at High RPM: Main jet blockage or a tear in the diaphragm.
Hanging Idle: Air leak at the intake boots or an overly lean mixture.
By following this service guide, you can restore the factory performance of your Mikuni BDS26 carburetors and ensure your motorcycle stays reliable for years to come.
Mikuni BDS26 is a constant velocity (CV) carburetor frequently found on motorcycles like the Yamaha XJ600 Seca II Virago 250 Hyosung GV250
. Maintaining these requires precise specifications for float height and jetting to ensure smooth throttle response and proper air-fuel mixtures. Technical Specifications According to service data for the Hyosung GV250 Yamaha XJ600 , standard settings are as follows: Specification (GV250) Specification (XJ600 USA) Idle Speed 1,450 – 1,550 RPM 1,200 – 1,300 RPM Float Height 6.2 – 8.2 mm Fuel Level (Wet) 4.0 – 6.0 mm below gasket line #90 (Front) / #87.5 (Rear) Jet Needle 2nd groove Fixed/Non-adjustable
Note: Float height measurement for the XJ600 is taken from the gasket surface to the top of the float while it is just touching the needle but not compressing the spring. Rebuild and Service Steps Mikuni BDS26 Carburetor Specifications | PDF - Scribd
If you have landed on this page, you are likely staring at a vintage carburetor with “Mikuni BDS26” stamped on the side. Whether it is bolted to a classic Arctic Cat snowmobile, a vintage Yamaha Enticer, or a custom go-kart, one truth remains universal: you cannot properly service this carburetor without a dedicated Mikuni BDS26 service manual.
In this article, we will explore everything you need to know about locating, using, and understanding the BDS26 service manual. We will cover common issues, exploded parts views, torque specifications, jetting charts, and step-by-step maintenance procedures that are typically found only in the official documentation.