Overview & Technical Specifications

The 1987 Mariner 20 HP is a highly regarded, twin-cylinder two-stroke outboard motor. Produced during an era of transition and collaboration for the Mariner brand, this engine is known for its impressive power-to-weight ratio, mechanical simplicity, and rugged durability on freshwater and saltwater utility boats alike.

Common Maintenance & Tune-up Specs

Keeping a vintage 1987 Mariner running smoothly requires adhering to precise tolerance limits. Below are the key maintenance values, ignition parameters, and fluid capacities required for seasonal tune-ups.

Ignition System Specs

• Spark Plug Type: NGK BP8H-N-10 (alternatively NGK BPZ8H-N-10 for radio noise suppression)
• Spark Plug Gap: 0.040 inches (1.0 mm)
• Spark Plug Torque: 20 lb-ft (27 N·m)
• Points/Breaker Gap: Not Applicable. This outboard utilizes a solid-state, breakerless Capacitor Discharge Ignition (CDI) system. There are no points to gap or maintain.

Cooling System & Water Pump

• Impeller Part Number: Quicksilver / Mercury OEM Part: 47-89983T (or 47-89983Q1)
• Service Interval: Inspect annually; replace every 2 years or 100 operating hours. Always replace the impeller plate and gaskets during housing disassembly.
• Cooling Indicator: Visually confirm water is exiting the “tell-tale” stream within 15 seconds of startup.

Gearcase & Lower Unit Lubrication

• Gear Lube Type: High-Performance Marine Gear Lubricant (SAE 90, API GL-5 rated)
• Gearcase Capacity: Approximately 9.0 fluid ounces (266 ml)
• Drain/Fill Plug Gasket: Always replace the dynamic non-asbestos fill/drain gaskets (OEM Part: 12-191833 or equivalent) at every oil change to prevent water intrusion.

Notable Features, Design & History

The Mariner-Mercury Connection

By 1987, the Mariner brand—originally established by Brunswick Corporation as a joint venture leveraging both Yamaha and Mercury manufacturing capabilities—had matured. While early Mariner portables (specifically sub-15 HP models of the late 70s/early 80s) were built primarily by Yamaha in Japan, the 1987 20 HP model is a direct cousin of the legendary USA-built Mercury 20 and 25 HP powerheads of the same era. This means it features domestic SAE thread fasteners, standard Mercury-designed ignition components, and enjoys vast aftermarket parts availability today.

Key Engineering Highlights

• Loop-Charged Induction: Unlike older cross-flow engines, the loop-charged cylinder design directs fuel-air mixture in a precise loop, creating more efficient combustion, better fuel economy, and lower carbon buildup.
• CDI Electronics: The Capacitor Discharge Ignition system provides reliable, high-voltage spark throughout the RPM range, making the engine highly resistant to spark plug fouling, even during extended periods of low-speed trolling.
• Lightweight Portability: Weighing around 112 pounds, it sits at the upper limit of true “portable” outboards. It is a favored choice for small fishing boats, aluminum skiffs, and inflatable tenders, offering a power-to-weight profile that modern four-stroke outboards struggle to match.

Common Troubleshooting Tips

If you encounter running issues with this model, focus on these three common areas:

1. Carburetor Jetting: Modern ethanol fuels degrade the fuel lines and varnish the small jets within the single carburetor. If the engine hesitates or requires partial choke to run, a carburetor teardown and ultrasonic cleaning is typically required.
2. Fuel Pump Diaphragm: The vacuum-operated fuel pump diaphragm can weaken or develop micro-tears over time. Replacing this inexpensive rubber diaphragm (located on the side of the engine block or integrated into the carburetor) is highly recommended every 5-7 years.
3. Grounding Wires: Ensure the ground straps between the CDI powerpack, coils, and engine block are clean and free of corrosion. Poor grounding is the leading cause of intermittent spark issues on these models.