Mercury Outboard Runs Too Rich

Mercury-Mariner outboard runs too rich refers to when the vaporized fuel mixture in an internal combustion engine contains a high proportion of fuel. It means the boat motor is receiving too much fuel. Running rich is not good for the outboard motor but it should still crank and drive when it’s running too rich. The outcome of running too rich is certainly better than an outboard running too lean.

If the Mercury outboard runs too rich it will create more exhaust smoke and foul the plugs. If it runs to rich the outboard motor will get poor gas mileage, slow acceleration and there might be a strong smell of gasoline while the engine is idling. Here are a few things that will cause your Mercury-Mariner outboard motor to run too rich, and how to fix it.

Mercury Outboard Runs Too Rich

ENGINE RUNS TOO RICH
Problem Solution
Fuel level too high. Adjust carburetor float level.
Carburetor floods. Replace worn inlet needle, punctured float or incorrect float settings. Remove dirt and particles that are preventing the inlet needle from seating properly.
Clogged idle nozzle air holes. Use compressed air to clear it blockage.
Air flow restricted. Check the air inlet on cowl for restrictions. Be certain nothing is obstructing the carburetor inlet.
Loose main fuel jet. Tighten jet.
Crack in fuel outlet tube (located in fuel tank). Replace outlet tube.
Anti-Siphon valve restricting fuel flow. Anti-siphon valve clogged with debri, vales too small or they may have too heavy of a spring.

Don’t just check the fuel system. Sometimes a person might get the impression that a problem exist within the fuel system, when in fact, it could be something else causing the issue.

In some instances extreme changes in humidity or temperature and/or elevation can result in a rich fuel mixture at WOT. Typically, every 1,000 foot increase of elevation change, the main jet size should be reduced by 2 percent.

Model, Year Application

The information on this page is specific to the following model, year Mercury-Mariner outboard 3-cylinder and 4-cylinder motors. This includes 75XD, Marathon and Seapro models.
YearModelCountrySerial Number
1987-199370hp, 75hpUnited States
Belgium
Canada
S/N 0A996142 thru 0D283221
S/N 09502135 thru 09793576
S/N 0A722297 and Above
1987-199380hp, 90hpUnited States
Belgium
Canada
S/N 0A996142 thru 0D283221
S/N 09502135 thru 09793576
S/N 0A722297 and Above
1988-1993100hp, 115hpUnited States
Belgium
Canada
S/N 0B209468 thru 0D283221
S/N 09523034 thru 09793576
S/N 0A731673 and Above

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