The British Seagull® Outboard Amal 416 Carburettor Main Jet injects the correct amount of fuel into the carburettor and sits in the bottom of the jet housing.
When replacing the carburettor main jet it is usual practice to exchange like for like. For example, if the main jet has '95' stamped on it, then replace with Main Jet No. 95 (CBN04022).