 Constant vacuum carburetors, also called variable choke carburetors and constant velocity carburetors, are carburetors where the throttle cable was connected directly to the throttle cable plate.Pulling the cord caused raw gasoline to enter the carburetor, creating a large emission of hydrocarbons.The Constant Velocity carburetor has a variable throttle closure in the intake air stream before the accelerator pedal operated the throttle plate.