1987 Merc Mariner 20HP Service Manual

The 1987 Merc Mariner 20HP is a two-cylinder outboard motor with 20 horsepower, featuring CD ignition system, 12-amp alternator, thermostatically controlled cooling, single carburetor, gear shift (forward-neutral-reverse), and electric start. This manual covers safety procedures, maintenance schedules, troubleshooting, repair procedures, specifications, and parts identification for the 1987 model year.

Specifications

Technical specifications for the 1987 Merc Mariner 20HP outboard motor.

Safety Precautions

Essential safety procedures for working on and operating the outboard motor.

1. Disconnect spark plug wires before servicing to prevent accidental starting.
2. Work in well-ventilated area when handling fuel.
3. Use proper lifting equipment - motor weighs approximately 95 lbs.
4. Wear safety glasses when working with batteries or chemicals.
5. Always use marine-grade parts and fluids.

WARNING! Gasoline is highly flammable. No smoking during fuel system service. Always have fire extinguisher nearby.

Maintenance Schedule

Recommended maintenance intervals for optimal performance.

1. Every 100 hours or annually: Change gear lube, inspect water pump.
2. Every 200 hours: Replace spark plugs, fuel filter.
3. Every 300 hours: Clean carburetor, check thermostat.
4. Annually: Grease fittings, check battery, inspect prop.
5. Every 2 years: Replace water pump impeller.

NOTE: More frequent maintenance required in saltwater or heavy use conditions.

Fuel System

Carburetor service and fuel system maintenance procedures.

• Carburetor: Single barrel, fixed main jet
• Fuel Pump: Diaphragm type
• Fuel Filter: In-line paper element
• Fuel Lines: Check for cracks and deterioration

Service Procedure: Remove carburetor, disassemble, clean with carburetor cleaner, inspect float level, reassemble with new gaskets.

Ignition System

CD ignition system testing and service.

Components: CD module, charge coil, sensor coil, spark plugs (Champion L77JC4), spark plug gap: 0.030 in.

Testing: Use DVA meter to check CD module output (should be 25-30 kV). Check resistance values: charge coil 650-850 ohms, sensor coil 9-13 ohms.

Timing: Set at 4° BTDC at idle, 21° BTDC at WOT.

Cooling System

Water pump and thermostat service procedures.

1. Drain plug: Remove to drain water from powerhead.
2. Water pump: Replace impeller every 2 years or if overheating occurs.
3. Thermostat: Opens at 143°F (62°C). Test in hot water.
4. Tell-tale: Should have strong stream at idle and above.

Tip: Always run motor in test tank or on boat when testing cooling system.

Lower Unit

Gearcase service and propeller maintenance.

Gear Lube: Use premium marine gear lubricant. Capacity: 11 oz.

Propeller: 3-blade aluminum, 10-1/4" diameter. Check for damage and replace if bent.

Shift Mechanism: Mechanical linkage. Adjust shift rod height if shifting problems occur.

Drain/vent plugs: Replace washers annually to prevent water intrusion.

Electrical System

Charging and starting system service.

Testing: Check battery voltage (12.6V static), charging voltage (13.5-14.5V at 1500 RPM).

Troubleshooting

Common problems and solutions for the 20HP motor.

Torque Specifications

Critical fastener torque values for reassembly.

• Cylinder head bolts: 180 in-lbs (in sequence)
• Spark plugs: 180 in-lbs
• Propeller nut: 55 ft-lbs
• Gearcase drain plugs: 60 in-lbs
• Flywheel nut: 45 ft-lbs
• Mounting bolts: 25 ft-lbs

NOTE: Use anti-seize compound on spark plugs and drain plugs.

Parts Diagrams

Reference diagrams for major assemblies. Contact Mercury Marine for original part numbers.

Major assemblies covered: Powerhead, carburetor, ignition system, lower unit, water pump, electrical system.

Parts Sources: Mercury Marine dealers, marine parts suppliers. Use serial number for exact parts matching.

Serial Number Location: On mounting bracket or freeze plug on powerhead.