Well going t be pretty hard to tell from here. You need to check if the engine is running on BOTH cylinders. The "flap inside the carb opens" indicates it is starving for fuel.

Did you reconnect the fuel pump correctly, vacuum in the bottom, fuel line in the side. Did you get the little springs back in the fuel pump and in correctly? Is the fuel line from the tank to the fuel pump clear? Is the gas tank cap vent clear?

You need to do some more checking and post the results.

Walt Conner.