How to keep fuel up to the curb after shut off.

how long is a while? if this is after days its the gas/ethanol evaporating out the carb. If this is after 20 min or an hour its hot start fuel boiling/evaporating out of the carb. Even if the line drains back and you dont have one of the two above problems there is usually enough fuel in the bowl of the carb to start and run it long enough for the pump to prime.