I have a 2000 90HP mercury 2 stroke oil injected outboard...I took the motor cover off and this was sitting in the bottom of the lower cowling under-neath carbs...it has a one way valve in it you can blow through it but it closes when you suck on it...Looks to be a vaccumn line of some kind, the lip on top looks to be where 2 pieces are put together to hold it...When I had my carbs cleaned last time I think this was not put back on..but I cannot see where it goes..I have attached a pic.
