The pressure in the fuel tank due to the lack of a vent is overriding the needle valve in the carburetor, allowing the gas to drip down the intake and flood the engine. It's also possible there's a pinhole in the fuel pump diaphragm that's dumping gas directly into the crankcase. All tanks have some sort of vent, either one that's open to the atmosphere, or with a charcoal canister in the vent line. Find the restriction and fix it, and replace the fuel pump if you think it's bad.
An electric pump conversion is a guaranteed flood situation, unless you install an oil pressure sensing switch and some other safety circuits along with it.
Jerry
