I have a 71 Chevelle with a L6 250. I changed my carburator to a rebuild Rochester 1 barrel (from RockAuto) because my old one was screwed up. Since I did this, my car is hard to start when it cold. I need to push a lot on the gaz pedal during I'm starting and after 40 sec it decide to start. Strangly, when it's hot, I can easily start it up normally. It look like the gaz has some difficulty to going up into mu carb. I don't know if it's a carburator ajustment problem or something else? Any idea???

Check if the choke is closed all the way. It won't start right away if the choke is slightly open when it's cold.

Thx for you quick answer, I will check this tonight. But, are you thing that my carburator could also be maladjusted? I have no idea how we tune a carburator.

The carburator is set up for low emission. All you have hot and cold idle adjustment screws and air / fuel mixture adj. If adjusted properly you should not have any problem. Make sure the choke is closed when it's cold. It cracks open once the motor is running.