To add to that, basically the air rushing through the gap between the throttle plate and the holes in the bottom of the carburetor throat creates a vacuum for a given airflow, the greater the airflow the greater the vacuum. I say vacuum, it’s more proper to say the atmospheric pressure is less in the carburetor throat than in the float bowl. That forces fuel up through the various jets and passages, and 34mm into the throat of the carb. The lower the fuel level, the longer this takes. It’s not instantaneous, which is not why the accelerator pump is there. The purpose of the accelerator pump is to deal with quick increases in the throttle position, which affect the vacuum signal and reduce the fuel flow as a result.
Anyway, here’s a link that might help:
Mercedes Benz Zenith Carb Manual – JaimeKop.com