Mikuni Bs26 Carburetor Manual Verified [updated] Jun 2026

: Engine overheats, backfires, or requires choke even when warm; spark plugs look white.

If the bike runs sluggishly between 1/4 and 3/4 throttle, adjust the needle clip. Lowers the needle (leaner). Move the clip down: Raises the needle (richer). 3. Cleaning and Rebuilding the BS26 A filthy carburetor is the #1 cause of poor performance.

Remove the two or three screws securing the top vacuum cap. Remove the cap slowly to prevent the internal spring from launching out.

Cause: Stuck float needle, debris in the needle seat, or a leaking float that has filled with fuel. Fix: Clean the needle seat or replace the float assembly. mikuni bs26 carburetor manual verified

: If you need more than 2.5 turns out, your pilot jet is too small. Less than 1 turn out → pilot jet too large.

Improper hose routing creates vacuum leaks or fuel starvation. Check your plumbing against this verified layout:

Understanding the four primary circuits of the BS26 makes troubleshooting and tuning highly intuitive: : Engine overheats, backfires, or requires choke even

Here are the key specifications of the Mikuni BS26 carburetor:

| Specification | Verified Value | Notes | | :--- | :--- | :--- | | | #127.5 (Left) / #130 (Right) | GS500 only. Singles use #125. | | Pilot Jet | #40 | (Some models use #37.5 – check your needle) | | Pilot Air Screw | 2.5 to 3.0 turns out | From fully seated. Do not exceed 4 turns. | | Float Height | 13.0 mm (13.7 mm for GN250) | Measure with carb inverted, float tang just touching needle spring. | | Needle Clip Position | 4th groove from top | (Standard is middle, but GS500 uses richer 4th) | | Idle Speed | 1,200 RPM (+/- 100) | Must be set when engine is fully hot. | | Diaphragm Part # | 13258-02C00 | (OEM Mikuni – Aftermarket fails quickly) | | Float Bowl Gasket | 13269-01A00 | Not reusable after soaking in cleaner. |

The bike, a worn-down Yamaha, had been sputtering for weeks. Every time Elias twisted the throttle, the engine gasped, starved of the perfect air-fuel ratio. He knew the culprit: the , a Constant Velocity (CV) carburetor known for its reliability—if you can find the right specs for it. The Search for the "Truth" Move the clip down: Raises the needle (richer)

| Symptom | Common Forum Advice | | | :--- | :--- | :--- | | Hesitation at 1/4 throttle | "Turn the PAS to 4 turns." | Wrong. Replace the pilot jet (#40) and set PAS to 2.5 turns. 4 turns means the pilot jet is clogged. | | Backfire on decel | "You have a header leak." | Check the mixture screw O-ring. If it is flat or missing, you have an unmetered air leak. Replace the O-ring (Part # 13373-43580). | | Slide won't rise | "Buy a stronger spring." | Check the diaphragm vent hole. On the air filter side of the carb, there is a tiny 1mm passage. If clogged, the slide cannot sense vacuum. Blow it out with compressed air. | | Rich idle, black smoke | "Lower the needle clip." | Check the float needle seat. The BS26 has a removable seat with a built-in filter screen. If the screen is torn, debris holds the needle open. Inspect with a bright light. |

The jet needle features a clip that can be moved into different notches: Air screw adjustment Mikuni carbs only

The Mikuni BS26 carburetor consists of several key components, including: