Velocity stacks are sold each.

The length of this velocity stack is 50mm which promotes better torque and mid-range response.

Choose your length depending upon your clearances and tuning requirements.  Shorter lengths relate to less restriction and higher RPM power whereas the longer lengths smooth flow more and provide higher low RPM torque characteristics.  Lengths are available in 35mm and 50mm. 

Keihin velocity stacks improve the air flow into the carburetors by smoothing small eddy currents that develop near the intake bells of carburetors, especially during high intake velocity conditions.  This not only increases horsepower by improving combustion, but also increases throttle response and fuel atomization.  Fits Keihin FCR 28-33 flat slide carburetors.