no gas after sitting

It could be evaporation but it sound more as if the gas in the carburetor and in the gas lines is draining back into the gas tank. This is also a normal problem in older vehicles and is normally caused by dry rotted rubber gas lines and/or a pin hole size leak(s). A friend of mind just corrected this problem in her 1970 318 Mopar by replacing the gas line from the tank to the carburetor now her car can sit for weeks and she pumps the gas peddle a few time and it starts right up with out having to turn the engine over and over…
Hope this helps!
KP