If the choke is fully closed, the fast idle screw is on the high step of the cam. Once you pump it to set the choke do not hit the throttle. If the vacuum pull off (dashpot) is adjusted so it pulls the choke open to far, then the next time you hit the throttle it'll knock the fast idle cam down a notch or more. Are the bowls full when you go to start (electric pump?). If not the modern fuels evaporate quickly. The bowls may be empty. No electric pump? Crank the motor to fill the bowls. Then set the choke with one pump. It should start right up and fast idle.
Doug