Simple vacuum leak in the fuel line, like a cracked hardened rubber fuel line, loose hose clamp, bad gasket at the tank pick up, bad vapor separator, too low float level, weak accelerator pump or restricted accelerator pump passages. Shouldn't the fuel line stay primed all the way to the fuel pump and the carburetor float chamber(s) stay full from when the engine is shut off. Check the accelerator pump function before you attempt to start it the next time after it's been sitting it should pump plenty of fuel if it has fuel in the carb