Pull off the big rund neck on top of the metering unit (loosen the big jubilee clip first) then you'll see the flap. When the trottle is closed, the pressure inside the hose is the same as outside, the flap isn't open, so the car is just running with idle mixture. When you open the throttle slightly, the flap also will open and give the engine the amount of fuel it needs for the air passing through flap and throttle. The more you open the throttle, the more the flap will open also (when there are no air leaks between engine and flap!).
Just open all four clips on top of the air filter box, then you can take the metering unit with the plastic cap of the box off. When you turn it, you'll see how the flap is mounted underneath. It's just like old scales. Wenn the flap opens, the other side will go down, the rod inside the metering unit also will go down and give more fuel to the injectors. Usually the oil damps from the engine are used to get some oil to this mechanism. Maybe after many years standing around the oil there was getting thick. So try some WD40 or Caramba on the moving parts and don't hesitate to clean it all with brake cleaner or something else. Flap and funnel also for sure.
And as mentioned above - when the metering unit is working - there are some more parts that might cause problems. Warm-up regulator (controls fuel pressure) and the injectors.