I have a Carter AVS on my 390 AMC engine. When the car sits for a few days, it will not start - it will just crank all day. I then took off the breather and I manually give it the throttle by hand and look down through the carb I do not see any gas squirting. Its not getting gas...

I get a small cup of gas and pour it into the carb and it starts right up and stays running and I'm good to go.

The fuel pump is new, the line is clean, and there is a new filter. I see gas in filter too...so there is gas getting there. The carb is super clean and it was rebuilt a year ago.

Bizarre...could it be something like the needle/seat or float sticks and prevents gas from coming into the carb, but once the engine runs it shakes it loose, allowing gas into the bowl?