Carb Tuning

Carburetor & Engine

Carb Type Mikuni BST34SS and similar — common aftermarket upgrade for CB900, ZX-series Throttle Bore (mm) Stock: 34mm Ideal bore for this config: 28mm (carb slightly over-sized) Bore sizing: 225cc/carb at 7,500 RPM → 28mm ideal

Pilot Jet Main Jet Needle Jet Stock: #159 Fuel/Pilot Screw (turns out) Typical range: 1.5–3.5 turns out 2.5 turns = +0% pilot fuel add (neutral)

Engine Parameters

Displacement (cc) Cylinders # of Carbs Base VE (%) Peak VE RPM RPM of peak power — not redline Redline RPM

Intake & Exhaust

Intake Exhaust

Conditions

Altitude (ft) 0 = sea level Air Temperature (°F) Fuel Blend Affects fuel volume readout (not AFR model)

Symptom Report

No symptoms added. Use the button below to describe a problem area.

Jet Recommendations

Main Jet

#120 (+8)

current: #112

Pilot Jet

#42.5 (+0.5)

current: #42

Targets 13.0 AFR at WOT/7,500 RPM (main) and 14.5 AFR at idle (pilot).

Presets:

RPM: 7,500 9,000 max

Throttle: 50% 100% max

AFR 14:1

Fuel Supply 48.01 cc/min

Fuel Demand 51.71 cc/min

Pilot 0%

Needle 100%

Main 0%

Fuel Map — Throttle × RPM

Rich

Lean

12.5 13.5 14.7 16+

AFR at 7,500 RPM

Model estimate — not a substitute for a wideband O2 sensor

Too Lean (>15.5) Too Rich (<11.0) Target (12.8–14.0) Stoich (14.7:1)

Fuel Volume at 7,500 RPM

Supply (from jets) Demand (13.0 AFR)

Circuit Contribution at 7,500 RPM

Pilot Needle Main

CV Slide Position

How far the vacuum slide opens at the current RPM vs. throttle plate position. Dashed line = perfect linear (cable-actuated) response.