Plot hole: Although Marty rips the fuel line, not all the fuel would have drained out of the tank. Only the fuel that was already in the fuel line would have leaked out, meaning it could simply be patched, bled and the engine would run without a problem.
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Suggested correction: A cut fuel line could absolutely drain the tank. Even if not a direct route with respect to gravity, it could still create a siphon effect and drain the tank.