Mikuni Bds26 Service Manual Fix May 2026

Mikuni BDS26 is a 26mm constant velocity (CV) carburetor commonly found on motorcycles like the Yamaha XJ600 Seca II (Diversion) Hyosung GV250

📘 Refer to Appendix C for jetting curves based on altitude and exhaust type. Unapproved modifications to the vacuum port drilling will cause tunability loss. mikuni bds26 service manual

8. Troubleshooting Guide

• Common symptoms and causes:

  Check the needle valve tip. If it has a visible ring or groove, it won’t seal properly, leading to a leaking carb. Mikuni BDS26 is a 26mm constant velocity (CV)

  1. Cold Start (Choke): A manual plunger (enrichment circuit) restricts airflow and adds extra fuel. Unlike a choke plate, the Mikuni system uses a separate fuel circuit.
  2. Idle Circuit: Controlled by the Pilot Screw (Fuel/Air screw) and Pilot Jet. Supplies fuel at idle and low throttle openings (0–1/8 turn).
  3. Mid-Range: Controlled by the Needle Jet and Jet Needle. As the throttle is opened (1/8–3/4 turn), engine vacuum lifts the diaphragm, raising the needle.
  4. High Speed (WOT): Controlled by the Main Jet. At Wide Open Throttle (3/4–Full), the needle is fully withdrawn, and fuel flow is dictated solely by the main jet size.

  Float and Valve Removal:

  The Float System: Controls the fuel level in the bowl. It consists of the float, the needle valve, and the seat. If the fuel level is incorrect, the engine will run too rich (high level) or lean (low level). Common symptoms and causes: Check the needle valve tip