1986 Merc 90 Service Manual

The 1986 Mercury 90 HP outboard motor is a 3-cylinder, 2-stroke engine with 89.7 cubic inch displacement, featuring CD ignition system, 12-volt electric start, thermostatically controlled cooling, and single carburetor. This service manual covers maintenance procedures, troubleshooting, repair instructions, and specifications for proper operation and longevity of your Mercury outboard motor.

Specifications

Technical specifications for the 1986 Mercury 90 HP outboard motor.

Safety Precautions

Essential safety procedures for working on your Mercury 90 outboard.

1. Disconnect spark plug wires before servicing to prevent accidental starting.
2. Work in well-ventilated area when handling fuel or batteries.
3. Use proper lifting equipment - motor weighs 268 lbs.
4. Wear safety glasses when working with batteries or pressurized systems.
5. Ensure motor is securely mounted before starting.

WARNING! Gasoline is highly flammable - no smoking or open flames during service. Always disconnect battery before electrical work.

Maintenance Schedule

Recommended maintenance intervals for optimal performance.

1. Every 100 hours: Change gear lube, inspect spark plugs.
2. Every 200 hours: Replace water pump impeller, inspect fuel lines.
3. Annual: Fog cylinders for storage, check battery condition.
4. Every 2 years: Replace fuel filter, inspect wiring harness.
5. As needed: Clean carburetor, check propeller for damage.

CAUTION! Follow torque specifications precisely to avoid damage to components.

Fuel System

• Carburetor: Single barrel with fixed high-speed jet.
• Fuel Pump: Diaphragm type, engine pulse operated.
• Fuel Filter: In-line replaceable cartridge.
• Fuel Lines: Check for cracks and deterioration annually.
• Mixture: 50:1 Mercury Premium Plus oil to gasoline.
• Adjustments: Idle mixture screw - 1-1/4 turns out from lightly seated.
• Cleaning: Remove and clean carburetor every 200 hours.

Ignition System

CD ignition system components and service procedures.

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

Spark Plug: Champion L77JC4 gap 0.030". Stator Output: 180-220V AC. Trigger Resistance: 800-1100 ohms. Coil Resistance: Primary 0.5-0.7 ohms, Secondary 5-7K ohms. Timing: 4° BTDC at idle, 21° BTDC at WOT.

Cooling System

Thermostatically controlled freshwater cooling system.

1. Water pump impeller replacement every 200 hours.
2. Thermostat opens at 143°F (62°C).
3. Inspect tell-tale stream for proper water flow.
4. Flush with freshwater after saltwater use.

Tip: Always run motor in water or with flush attachment to prevent water pump damage.

Lower Unit

Gearcase maintenance and service procedures.

1. Drain and refill gear lube every 100 hours.
2. Use Mercury High Performance Gear Lube.
3. Check for water contamination in gear oil.
4. Inspect propeller shaft seals for leaks.
5. Gear lube capacity: 22 oz (650 ml).
6. Shift linkage adjustment: Neutral safety switch engagement.
7. Bearing carrier removal for seal replacement.

WARNING! Do not operate motor without proper gear lube - will cause immediate damage.

Electrical System

12-volt electrical system components and testing.

Propeller

Propeller selection, installation, and maintenance.

Standard Propeller: 13" x 19" aluminum. Installation: 55 ft-lbs torque. Hub: Rubber cushion drive. Inspection: Check for nicks, dents, and fishing line around shaft. Replacement: Match pitch to boat load and performance requirements.

Storage & Winterization

Proper procedures for long-term storage.

1. Add fuel stabilizer to full fuel tank.
2. Fog cylinders through spark plug holes.
3. Change gear lube and inspect lower unit.
4. Disconnect battery and store fully charged.
5. Store in upright position on motor stand.

CAUTION! Do not store with ethanol-blended fuel for more than 30 days without stabilizer.

Troubleshooting

Technical Support: Mercury Marine Customer Service: 1-920-929-5040

Parts: Contact authorized Mercury Marine dealer for genuine parts