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Mercury/Mariner 25 MARATHON - User Manual

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MODELS
United States 0G044027 and Above. . . . . . .
Belgium 09807909 and Above. . . . . . . . . . .
With Serial Numbers
SERVICE
MANUAL
20 JET 20 25
Mercury/Mariner
25 MARATHON 25 SEAPRO
Printed in U.S.A.
1998, Mercury Marine
90-826883R2 JUNE 1998

Table of Contents

Questions and Answers

  • J
    Jamie SantiagoAug 15, 2025
    Why does my Mercury/Mariner 25 MARATHON engine misfire at high RPM?
    • C
      Courtney MorganAug 15, 2025
      Engine misfiring at high RPM can be caused by a defective coil or a defective switch box. You should check or replace the defective coil and/or switch box to resolve this issue.
  • D
    Dana YoungAug 18, 2025
    What to do if my Mercury/Mariner Outboard Motor engine starts hard when cold?
    • M
      Michael SanchezAug 18, 2025
      If your engine is hard to start when cold, it could be due to a defective trigger assembly or a defective ignition switch box. Check the trigger assembly and ignition switch box to resolve this.
  • R
    robertskarenAug 22, 2025
    What causes no spark or weak spark on both cylinders of my Mercury/Mariner 25 MARATHON Outboard Motor?
    • K
      kimberly39Aug 22, 2025
      If there's no spark, the cause may be the trigger, stator, ignition switch box, or a bad ground connection from the switch box to the block. If there's a weak spark, it could be the stator. Check the trigger, stator, ignition switch box, or ground connection. If the spark is weak, focus on checking the stator.
  • D
    dylan16Aug 26, 2025
    What causes my Mercury/Mariner 25 MARATHON to start hard when hot?
    • C
      Cheryl BrownAug 26, 2025
      A defective switch box or trigger can cause your engine to start hard when it is hot. Check or replace the defective switch box or trigger.
  • O
    onealkristenAug 28, 2025
    What causes timing fluctuations in my Mercury/Mariner 25 MARATHON?
    • J
      John EricksonAug 28, 2025
      Timing fluctuations can be caused by a shorted trigger wire or a defective ignition switch box. Check the trigger wire or replace the ignition switch box to fix this.
  • S
    Stephen HallSep 1, 2025
    What to do if my Mercury/Mariner Outboard Motor engine misfires at low RPM but runs smooth at high RPM?
    • T
      Tina Robbins DVMSep 1, 2025
      If your engine misfires at low RPM but runs smoothly at high RPM, it could be due to a defective harness or loose connections, a defective switch box, or a defective stator. Check the harness and connections, switch box, and stator.
  • J
    James MooreSep 4, 2025
    Why won't the timing advance on my Mercury/Mariner 25 MARATHON Outboard Motor?
    • S
      Sheila SheltonSep 4, 2025
      If the timing will not advance, it is likely due to a defective switch box. Replace the switch box to resolve this.
  • D
    Douglas PeckSep 7, 2025
    What causes no spark or weak spark on one cylinder of my Mercury/Mariner Outboard Motor?
    • S
      Stephen CampbellSep 7, 2025
      If you have no spark or a weak spark on one cylinder, the cause could be the ignition switch box or the coil. Check the ignition switch box or coil.
  • D
    Dr. Lauren Davis MDSep 12, 2025
    What to do if my Mercury/Mariner 25 MARATHON Outboard Motor occasionally misfires?
    • M
      michael12Sep 12, 2025
      If your engine occasionally misfires, replace the standard spark plug with an inductor plug. Also, check for a bad ground connection from the switch box to the block. Replace the spark plug with an inductor plug and check the ground connection.

Summary

Important Information

Specifications

Details specific technical data and parameters for the outboard motor models.

Maintenance

Outlines routine inspection and maintenance tasks for the outboard motor.

Outboard Motor Installation

Covers the procedures and safety for installing the outboard motor onto a boat.

Electrical

Ignition

Details the ignition system components, specifications, and troubleshooting for engine starting.

Battery Charging and Starting Systems

Explains the battery, charging system, and starting system components and maintenance.

Wiring Diagrams

Presents various wiring diagrams for different models and systems of the outboard.

Fuel System

Carburetor;Fuel Pump

Details specifications, adjustments, and components of the carburetor and fuel pump system.

Emissions

Explains exhaust emissions, standards, and control methods for marine engines.

Powerhead

Specifications

Lists specifications for engine components like bore, stroke, pistons, and crankshaft.

Powerhead Removal

Outlines the steps for safely removing the powerhead from the engine assembly.

Powerhead Reassembly

Provides instructions for reassembling the powerhead after disassembly or repair.

CLAMP and SWIVEL BRACKET

Clamp Bracket Assembly

Details the components and assembly of the clamp bracket used for mounting the outboard.

Swivel Bracket Assembly

Explains the assembly and components of the swivel bracket, critical for steering.

Gear Housing

Specifications

Lists specifications for the gear housing, including ratio, capacity, and lubricant.

Gear Housing Removal

Outlines the steps for safely removing the gear housing from the outboard.

Reassembly

Provides instructions for reassembling the gear housing after disassembly or repair.

Jet Drive

Specifications

Lists specifications for the jet drive system, including impeller clearance.

Outboard Mounting Height

Explains how to set the correct outboard mounting height to prevent cavitation or drag.

Shift Link Rod Adjustment

Details the procedure for adjusting the shift link rod to ensure proper gear engagement and safety.

Throttle;Shift Linkage

Throttle and Shift Linkage (Tiller Handle Shift)

Details the components and assembly of the throttle and shift linkage for tiller handle models.

Control Platform

Covers the removal, disassembly, reassembly, and installation of the control platform.

Horizontal Shift Shaft

Covers the removal, installation, and securing of the horizontal shift shaft.

Tiller Handle

Tiller Handle Assembly

Details the components and assembly of the tiller handle, including various models and colors.

Tiller Handle Removal;Disassembly

Outlines the steps for removing and disassembling the tiller handle assembly.

Side Shift

Throttle And Shift Linkage (Side Shift)

Details the components and linkage for throttle and shift operations on side shift models.

Throttle Cam;Control Lever

Covers the reassembly and installation of the throttle cam and control lever for side shift models.

Manual Starter

Specifications

Provides specifications for the manual starting system, including rope length.

Starter Assembly (Manual)

Details the components and assembly of the manual starter system.

Rewind Starter Assembly

Outlines the removal and installation procedures for the rewind starter assembly.

Starter Rope Replacement

Provides instructions for replacing the starter rope on the rewind starter assembly.

Mercury/Mariner 25 MARATHON Specifications

General IconGeneral
BrandMercury/Mariner
Model25 MARATHON
CategoryOutboard Motor
LanguageEnglish