Purpose Air Cut Valve ?

[aSTerix]

Can anyone shed light on the purpose of the Air Cut Valve ?

 

[bdalameda]

These cut off the fuel supply when the throttle is closed rapidly when the vacuum in the intake spikes. Prevents sudden over-fueling, reduces emissions and backfires.

 

[afrymoyer]

On the ST Keihin, the "air cut" circuit is exactly that. It cuts the AIR going through slow speed/pilot circuit, purposely over fueling the circuit during instances of high vacuum. At a higher rpm throttle roll off to idle, the fuel mixture goes extremely lean. Again, at high vacuum, the air cut diaphragm pulls the air plungers CLOSED and blocks air, eliminating atomization and enriching the circuit. This allows the engine to have the proper mixture and reduce fuel starvation. All of the jetting can't keep up during those instances of extremely high vacuum, so they "cut the air".