Mercury Mariner Outboard 225 3 Litre Service Manual 1994, Page 112Get this manual
Testing Fuel Pump Install clear fuel hose(s) between fuel pump and carburetor(s)Run engine, and inspect fuel passing thru
hose(s) for air bubblesIf air bubbles are found, see "Air
Bubbles in Fuel Line," belowIf air bubbles are NOT foundsee uLack of Fuel Pump Pressure ," below
Troubleshooting Fuel Pump
PROBLEMAir Bubbles in Fuel Line
Low fuel in tankFill tank with fuelCheck and tighten ll connectors