The effects of OLD gasoline in a classic car

Is the electric choke hooked up? If yes, these need adjusted to increase or decrease the speed as to which they come off choke.

For either the Holley or the Demon, was the high idle in use?

I have experienced this with my truck, a mild built 460 Ford with a 3310 Holley. It sits a lot, it will run somewhat poor until I fill the tank then its back to normal.

I'm still using gas in my lawn mower that was in my Dart 5 years ago, and it sits in my shed in a sealed container. The mower is hard to start, but its older, once it fires it don't quit until I do.