I have to crank for a long time to start the engine in my 70 Charger after it's been sitting for a few days. Why do the gasolin evaporate or return to the fuel tank? Running stock fuel tank and 5/16" fuel line, brand new Carter mechanical fuel pump, Edelbrock fuel filter before an brand new Edelbrock 800cfm carb.

It's most likely just evaporating. Todays fuel blends are HIGHLY evaporative and will often leave a carb dry after a couple days of sitting. The only easy solution to starting a dry carb (cranking forever) is to add an electric fuel pump. Problem solved.

is it possible to add an electric pump only to only fill up the bowls and then switch it off and run the mechanical pump? will the mechanical pump be able to such the gasolin trough the electric pump?

Probably but not something I've messed with, maybe post in the race section with that question.

yes, that will work. i've done this on several cars using a basic carter 5lb pump available at the parts store. mount it close to the tank and lower than the sending unit outlet.

I put in an electric pump and remove the mechanical pump. Wire it into the key so I put the key in the run position, let the pump run for 5 seconds or so to fill the carb then pump the throttle and hit the key and she fires. I don't even mess with the mechanical pumps anymore.

Some have suggested having the electric pump before the mechanical pump could allow the electric pump to blow through the diaphragm and push fuel into the oil and the restriction of the electric pump when shut off could cause the mechanical pump to struggle too much to draw fuel. I don't know, I never understood why you would want to run two pumps when one will do.

I have the same problem when sitting after about 3 weeks.

Thanks for the comments, sounds like an electric pump would solve the problem. what electric pump would hold up long term?

Non ethanol fuel greatly helps this problem. And many others.

I have a Carter P4594 that I have not put in the car yet. It does not need a regulator. https://www.summitracing.com/parts/crt-p4594