The oil is so thick it will trap air in the case and not be full. You fill from the bottom til it squirts ou the top. If you put the top plug back in before pulling the pump off, that will airlock the case and very little will come out the bottom hole. Or you can turn the motor
on its side or almost upside-down before disconnecting. That might not be good for a 4 stroke
, but a 2 stroke won't care.