I don’t think the bottom crank seal is the issue, even though the pieces might be messed up. Keep in mind that this engine has the overboard drain system, so excess fuel from a flooded crankcase will be directed toward that drain and down and out of the exhaust housing/gearcase. OK, so it seems like you have more or less ruled out the carburetor as the issue. So, it seems like the intake manifold is the next logical step. Again, you are sure that the fuel air hoses are connected properly? Keep in mind that if reversed, the fuel line will be directed to the intake manifold and will just fill the crankcase when primed.
Is this motor new to you? If so, did it ever run correctly? You mentioned that it appeared that the intake manifold had been off before. If the engine is new to you, and has never run for you, then I would expect that something is messed up in there…Perhaps someone was trying to convert it to a single line/fuel pump unit. Perhaps the black rubber pressure valves are broken/messed up, maybe the intake gaskets are reversed….It only takes a few minutes to pull the intake apart, do you have new gaskets? Oftentimes the gaskets will come off clean, provided no one has used sealer on them….
