Two things come to mind for that, the carb must be in good operating shape and the impulse line needs to be connected tight to the cylinder base and the nipple on the carb. The carb pumps fuel by the impulse signal from the crankcase, connections need to be airtight, passage to the crankcase must be clear. When using sealers between cylinders and the base it is easy to block the impulse channel , seen it many times by now. The carb must be functioning proper meaning good tight diaphragms, tight but not hard, cannot be stretched and wrinkly to pump fuel. I spray a few pumps of mix I have in a spray bottle through the carb throat to get the first startup, sometimes it takes a cycle or two to pull fuel through and fill the fuel bowl in the carb. If the engine will start and run a few seconds you know it has spark and compression, try a few cycles of spray through the carb, pull over and let the engine start and run, after 3-4 cycles of this and it will not keep running then further investigation is required to find why its not pulling fuel. Fuel line to the tank must also be air tight at the carb and all the way into the tank down at least below fuel level.