Things to look at: Is the float set correctly in the carb? Is the fuel filter clean, and fresh gas in the gas tank? Have you looked for vacuum leaks and broken vacuum hoses? Are you sure the carb is getting enough gas to begin with? You can pull the main gas line from the carb, and crank the engine. If gas just drizzles out, the fuel pump may have issues. The fuel pump has a rubber diaphragm in it, which can rot out over time, especially using ethanol-laced gas. You should get a strong stream, with a heartbeat-like pumping action, as it's a mechanical fuel pump. Check the integrity of the fuel line coming out of the fuel tank, as they can get rusted, so you suck 1/2 air, 1/2 fuel.
