[Gaz64]

Patrick,

Your secondaries shouldn't open all the way unless the carburettor is sized EXACTLY correct for the engine.

Example would be 850 VAC Secondary on a 300 inch engine.
The engine would have to spin to about 9800rpm to have the secondaries even close to fully open.
This doesn't take typical volumetric efficiency into account.

Your car goes harder because it is pulling on the boosters harder.

Don't worry about wondering whether they open fully, if they do then at some point the engine is not receiving the fuel that the carb was designed to deliver - fuel delivery curve.
Hook up a vacuum gauge and roadtest again.
Observe WOT vacuum, should be between .5 and 1.5 inches. Less than .5 - carb is too big, or secondary spring too light. More than 1.5 - carb too small or spring too heavy.

Changing from a restrictive air filter proves your own point of the air cleaner was restricting power. The secondaries open sooner as a consequence of a higher venturi vacuum signal across ALL 4 barrels.

Thoughts?