1987 115 hp 6 cylinder mercury outboard manual

The 1987 Mercury 115 hp 6-cylinder outboard engine is a classic marine powerplant featuring a 99.8 cubic inch displacement, loop-charged induction system, Thunderbolt CD ignition, through-hub exhaust, and power trim/tilt. This manual covers operation, maintenance, troubleshooting, and repair procedures for this reliable two-stroke outboard motor. Below are key sections covering specifications, operation, maintenance, troubleshooting, and repair procedures.

1. Specifications 2. Operation Instructions 3. Maintenance Schedule 4. Fuel System 5. Ignition System 6. Cooling System 7. Lower Unit 8. Electrical System 9. Storage & Winterization 10. Troubleshooting 11. Parts & Service

Specifications

Technical specifications for the 1987 Mercury 115 hp 6-cylinder outboard engine.

Specification	Details
Horsepower	115 hp @ 5000 RPM
Cylinders	6 in-line
Displacement	99.8 cubic inches (1634 cc)
Bore x Stroke	3.00" x 2.36" (76.2mm x 60mm)
Ignition System	Thunderbolt CD
Fuel System	3-carburetor system
Fuel/Oil Ratio	50:1 Mercury Premium Plus or Quicksilver 2-cycle oil
Gear Ratio	1.85:1
Weight	Approximately 295 lbs (134 kg)
Starting System	Electric start standard

Operation Instructions

Proper starting, running, and shutdown procedures for the Mercury 115 hp outboard.

Pre-start check: Verify fuel level, oil mixture, battery charge, and cooling water intake.
Starting procedure: Turn key to ON, push in for choke (cold start), turn to START.
Warm-up: Allow engine to idle for 2-3 minutes before applying throttle.
Running: Monitor temperature gauge and warning horn; avoid prolonged trolling.
Shutdown: Return to idle, turn key to OFF position.

WARNING! Always provide adequate ventilation when running engine. Carbon monoxide poisoning can occur in enclosed areas.

Maintenance Schedule

Recommended maintenance intervals for optimal performance and longevity.

After first 20 hours: Change gear lube, check torque on mounting bolts.
Every 100 hours or annually: Replace spark plugs, water pump impeller, fuel filter.
Every 200 hours: Clean and sync carburetors, check compression.
Every 300 hours: Replace thermostat, inspect reed valves.
Annually: Fog cylinders for storage, check battery condition.

CAUTION! Always disconnect battery before performing electrical maintenance.

Fuel System

Carburetors: Three fixed-jet carburetors with choke system.
Fuel Pump: Diaphragm-type with check valves.
Fuel Filter: In-line replaceable element.
Fuel Recommendations: 87 octane minimum, ethanol-blended fuels acceptable with proper stabilizer.
Troubleshooting: Hard starting or hesitation indicates possible carburetor issues.

Ignition System

Thunderbolt CD ignition system with 6 individual coils and switch boxes.

Components: Stator, trigger, switch boxes, coils, spark plugs.

Spark Plug: Champion L77JC4 or NGK BUHW gapped at 0.040". Timing: Fixed at 21° BTDC at WOT. Troubleshooting: Use DVA meter to test components; typical stator output 150-200V AC.

Cooling System

Water pump impeller replacement and thermostat operation.

Water pump service: Replace impeller every 2 years or 200 hours.
Thermostat: Opens at 143°F (62°C); test in hot water.
Tell-tale stream: Should be strong and consistent at all RPMs.

Tip: Always run engine in test tank or with flush device when out of water.

Lower Unit

Gearcase maintenance and propeller specifications.

Gear lube: Use Mercury High Performance Gear Lube or equivalent.
Drain/fill: Remove both screws; pump lube from bottom until clean lube emerges from top.
Propeller: Standard 13-1/4" x 17" aluminum; various pitches available.
Shifting: Mechanical shift with clutch dogs; adjust shift cable if hard shifting occurs.
Water intake: Keep clear of debris; check for fishing line around prop shaft.

WARNING! Always check for water contamination in gear lube. Milky appearance indicates seal failure.

Electrical System

12-volt charging system and wiring diagrams.

Charging System: 9-amp alternator with rectifier/regulator. Battery: Minimum 450 CCA marine battery. Wiring: Color-coded harness with waterproof connectors. Fuses: 20-amp main fuse near starter solenoid.

Storage & Winterization

Proper procedures for short-term and long-term storage.

Fuel system: Add fuel stabilizer, run engine to distribute through system.
Fogging: Spray fogging oil into carburetors while running until engine stalls.
Lower unit: Change gear lube to check for water intrusion.
Battery: Remove and store fully charged in cool, dry location.
External: Flush with fresh water, wipe down, cover with breathable cover.

CAUTION! Never store engine with ethanol-blended fuel without stabilizer.

Troubleshooting

Symptom	Possible Cause	Corrective Action
Engine won't start	Fuel, spark, or compression	Check fuel delivery, spark on all cylinders, compression test.
Engine misses at speed	Ignition or fuel	Check spark plugs, stator output, carburetor sync.
Overheating	Water pump or thermostat	Replace impeller, check thermostat, clear water passages.
Poor fuel economy	Carburetion or prop	Clean carbs, check for proper WOT RPM with current prop.
No charging	Stator or rectifier	Test stator AC output, check rectifier diodes.

Compression: Should be 100+ PSI and within 10% across all cylinders.

Parts & Service

Genuine Mercury parts and authorized service locations.

Always use genuine Mercury or Quicksilver parts for optimal performance and warranty coverage. For technical assistance or to locate your nearest authorized Mercury Marine dealer, contact Mercury Marine Customer Service at 1-920-929-5040 or visit www.mercurymarine.com. Keep your engine serial number (located on mounting bracket) available when ordering parts.