Solexes are not hill-climbers. On steep hills, it's necessary to pedal to help the motor out.
It's important to make sure that the metal fuel pipe from the pump to the carb isn't touching the engine crankcase cover.
Check the filters in the carb and fuel tank to be sure they're not restricted. A restriction in the fuel pipes will also cause it to stall. They can be removed and cleaned with compressed air. I've already mentioned cleaning the fuel jet the same way.
Problems with the spark plug and/or wire will cause a stall also.