1987 Mariner 20HP Service Manual

The 1987 Mariner 20HP outboard motor is a reliable two-stroke engine designed for small to medium-sized boats. This service manual covers maintenance, repair, and troubleshooting procedures for the 20 horsepower model including specifications, lubrication systems, fuel system, ignition system, cooling system, powerhead, gearcase, and electrical components. Below are key sections for safety, specifications, maintenance procedures, troubleshooting, and repair instructions.

1. Specifications 2. Safety Precautions 3. Maintenance Schedule 4. Fuel System 5. Ignition System 6. Cooling System 7. Powerhead 8. Gearcase 9. Electrical System 10. Troubleshooting 11. Torque Specifications

Specifications

Technical specifications for the 1987 Mariner 20HP outboard motor.

Specification	Value
Horsepower	20 HP @ 5000 RPM
Cylinders	2
Displacement	20.9 cu in (342 cc)
Bore x Stroke	2.48 x 2.16 in (63 x 55 mm)
Ignition System	CDI (Capacitor Discharge Ignition)
Starting System	Manual pull start
Gear Ratio	2.15:1
Full Throttle RPM	5000-6000 RPM
Fuel/Oil Ratio	50:1
Weight	87 lbs (39.5 kg)

Safety Precautions

Important safety information for working on the Mariner 20HP outboard.

Always disconnect spark plug wires before working on engine.
Work in well-ventilated area when handling fuel.
Use proper lifting equipment - engine weighs 87 lbs.
Wear safety glasses when working with batteries or compressed air.
Disconnect battery before performing electrical work.

WARNING! Gasoline is highly flammable - no smoking or open flames near work area. Always have fire extinguisher available.

Maintenance Schedule

Recommended maintenance intervals for optimal performance.

Every 100 hours or annually: Change gearcase lubricant.
Every 100 hours: Clean and regap spark plugs.
Every 200 hours: Replace water pump impeller.
Every 300 hours: Decarburize engine.
Annually: Check and tighten mounting bolts.
Before storage: Fog engine and stabilize fuel.

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

Fuel System

Fuel Pump: Diaphragm type, engine pulse operated
Carburetor: Single barrel, fixed main jet
Fuel Filter: In-line paper element
Fuel Recommendations: 87 octane minimum, ethanol blends up to 10%
Oil Mix: TC-W3 certified two-stroke oil at 50:1 ratio

Ignition System

CD ignition system specifications and service procedures.

Spark Plugs: Champion L77JC4, gap 0.030"

Ignition Timing: Fixed at 28° BTDC at WOT

Flywheel: 84-tooth, requires special puller for removal. Stator output: 150V AC minimum at cranking speed. Trigger coil resistance: 130-170 ohms.

Cooling System

Water pump and thermostat service procedures.

Water pump impeller replacement interval: 2 years or 200 hours.
Thermostat: Opens at 143°F (62°C).
Pressure relief valve: Located in cylinder head cover.
Tell-tale stream: Should be strong at all operating speeds.

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

Powerhead

Cylinder block, pistons, crankshaft, and reed valve service.

Compression test: Should be 100-120 psi, within 10% cylinder to cylinder.
Reed valves: Check for cracks or improper seating.
Crankcase pressure test: Apply 7 psi - should hold pressure for 5 minutes.
Piston ring end gap: 0.010-0.020 in.
Crankshaft runout: Maximum 0.002 in.

WARNING! Always use new gaskets when reassembling powerhead components.

Gearcase

Lower unit service including propeller, driveshaft, and gears.

Gearcase capacity: 5.5 oz (165 ml) of 80W-90 gear oil. Propeller shaft seal replacement requires special tools. Forward gear backlash: 0.004-0.008 in. Water pump housing: Inspect for wear or scoring during impeller replacement.

Electrical System

Charging system, starter, and wiring diagrams.

Charging System: 6 amp alternator
Battery: 12V, minimum 35 Ah
Starter: Manual rewind only (no electric start option)
Stop Switch: Lanyard type safety switch
Wiring: Color-coded harness with waterproof connectors

Troubleshooting

Symptom	Possible Cause	Corrective Action
Engine won't start	Fuel, spark, or compression	Check spark, fuel delivery, and compression in that order.
Engine runs rough	Carburetor, ignition	Clean carburetor, check spark plugs and ignition timing.
Overheating	Water pump, thermostat	Replace water pump impeller, check thermostat operation.
No water from tell-tale	Blockage or pump failure	Clear tell-tale hose, replace water pump if necessary.
Gearcase noise	Gear wear, lack of oil	Drain and inspect gear oil for metal particles.

Service Note: Always use genuine Mariner parts for best performance and reliability.

Torque Specifications

Critical torque values for engine assembly.

Component	Torque Value
Cylinder Head Bolts	18-20 ft-lbs
Spark Plugs	18-20 ft-lbs
Flywheel Nut	45-50 ft-lbs
Propeller Nut	55-60 ft-lbs
Gearcase Drain/Fill Screws	40-50 in-lbs
Mounting Bolts	25-30 ft-lbs

Technical Support: Contact Mariner customer service for additional technical assistance.