How to Troubleshoot a 25 HP Mercury Boat Motor

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Many people find themselves troubleshooting a 25 HP Mercury boat motor after the boat has not been used for several months. By understanding how to troubleshoot the motor, a person can also understand what kinds of things they should avoid doing that may have damaged the motor in the first place.

Items you will need

  • Spark tester

  • Screwdriver

  • Carburetor cleaning solution

Concentrate on understanding if there is a spark in the motor and if the fuel is reaching the motor. These are the two things that are most often the culprits when there is a problem with a 25 HP Mercury boat motor. Although problems could come from other sources, it is unlikely.

Troubleshoot the ignition by using a spark detector. Auto supply stores sell these detectors. They do not need to be made specifically to detect the spark in a 25 HP Mercury boat motor. A generic spark detector will do. Set the gap on the detector to 3/8 of an inch. Crank the motor to see if there is a spark. Remember to check for a spark on both of the 25 HP Mercury boat motor cylinders. If there is a spark, there is a good chance that the ignition is not the problem.

Check the black and yellow wires if there is no spark. This can be done by removing it from the switchbox and replacing it with a new wire. If there is a spark after replacing the wire, the problem has been solved.

Troubleshoot the carburetor, if there is still no spark, by looking and feeling for any sticky gunk that is inside of the carburetor. Too much of this gunk can cause the carburetor to overflow. When an overflow in a 25 Hp mercury boat motor occurs, the motor can become flooded and make it difficult for valves to close.

Remove the fuel cap to drain the old fuel. Soak the carburetor in a carburetor-cleaning solution. Do not attempt to remove the main jet from the carburetor until the carburetor has soaked for at least an hour to remove the stickiness.

Remove the main jet from the carburetor by sticking a flathead screwdriver through a hole in the carburetor to dislodge the jet. Do not attempt to open the fuel jet with a piece of wire or metal. Instead, rinse and soak the jet and then spray the jet with compressed air. This will remove the gunk that was stopping the fuel from properly flowing.

Replace the jet inside the carburetor and put the carburetor back in the 25 HP mercury motor.


  • Do not leave fuel in the tank after the boating season has ended. This can increase the likelihood of problems occurring the next time the motor is used. Do not use starting fluid to test your motor. Never disconnect any wires when the ignition is switched to "on."


  • Check to make sure all of the hoses hoses and connections are tight before opening up the carburetor.
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