[Synchro909]

An air filter fitted to a Model A carby might restrict the air flow and reduce the pressure from there on right into the manifold. Because the float bowl is originally vented to the atmosphere, the pressure in there does not change when the filter is fitted. That leaves the pressure in the bowl higher than in the throat of the carby. That means more fuel is pushed through and the motor runs rich. By pressure balancing the carby, the pressure in the bowl is made the same as in the throat so no extra fuel is supplied.
If you have a balanced carby you should notice no difference compared to the original if you run with or without an air filter. The only time there is a problem is if you run a restrctive air cleaner without balancing the carby. An air filter with little restriction causes little difference.
Hope I explained that clearly enough!