🚀 go-pugleaf

This is an observation made on my '82 Toyota Corolla but may apply to
most other cars as well.

There is a noticeable change in engine performance whenever I fill up the
gas tank. The engine sounds harsher and the pickup suffers until the tank
is about half full. I don't think the weight of gasoline is the reason
since the car does not seem to object to extra passengers with a less than
half filled gas tank(full tank holds 13.6 gallons).   The mileage averaged over
half the tank is 2-3 mpg less than mileage averaged over 3/4 of the tank.
I also noticed that air rushes into the tank whenever I open the cap. There
was a posting sometime back about air not being drawn into the tank properly.
Could this be the problem with my car? How does the emptiness of the tank
affect the amount of air drawn in? What can I do to verify this? I have no
other problems(hesitation etc.) except the above degradation in performance.

G.A.Venkatesh