There are only 3 ways fuel can leave the carb, evaporation, draining back, or leaking at the bottom plugs. We pressure test and seal up the castings during rebuilding to prevent leakage. This new fuel doesn't stick around long, but I would think it would still be there overnight. Two ways to prevent drain back, install a filter with a check valve in it, or remove the tiny clip from the needle assembly.....Cliff