Secondary Throats Air-door Adjustment
In curcumstances where manifold vacuum is too low to begin drawing fuel through the secondary jets (eg: full throttle at low engine speed), the air-door will not open either, preventing any flow through the secondary throats, and preventing the engine bogging down through over-carburation.
In practice, there are two ways in which the air-door can be out of adjustment. The air-door spring can be under tensioned, allowing the door to open before fuel flow has started in the secondary jets; or it can be over tensioned, preventing the door from opening until sometime after fuel flow has started in the secondary jets. The first situation will result in a period of too-lean mixture, the second will result in a period of too-rich mixture. Both cases will cause a bog or stumble in the engine as the secondaries are opened.