1991 Mariner 15 HP Outboard Specs & Part Numbers
Technical Guide: 1991 Mariner 15 HP Outboard Motor
The 1991 Mariner 15 HP outboard motor is a highly regarded, twin-cylinder, two-stroke marine engine. Produced during an era when Mariner (a division of Brunswick Corporation / Mercury Marine) was known for producing highly reliable, lightweight, and easily serviceable portable outboards, this engine remains a popular choice for small fishing boats, utility crafts, and tenders. This technical guide provides comprehensive specifications, maintenance guidelines, and historical context for owners, mechanics, and restorers.
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1. Overview & Technical Specifications
The 1991 Mariner 15 HP is built on a proven two-stroke platform shared with its sibling, the Mercury 15 HP of the same vintage. It features a loop-charged induction system for efficient fuel-air mixing, a lightweight aluminum alloy block, and an integrated through-hub exhaust system to reduce operating noise.
| Specification | Value / Detail |
|---|---|
| Horsepower | 15 HP (11.2 kW) @ 5500 RPM |
| Engine Type | 2-Cylinder, Inline, 2-Stroke |
| Displacement | 16.0 cu. in. (262 cc) |
| Bore & Stroke | 2.375 in. x 1.812 in. (60.3 mm x 46.0 mm) |
| Fuel/Oil Ratio | 50:1 (Unleaded Gasoline to TC-W3 2-Stroke Outboard Oil) |
| Ignition System | Modular CD (Capacitor Discharge Ignition) |
| Full Throttle RPM Range | 5000 – 6000 RPM |
| Gear Ratio | 2.00:1 |
| Dry Weight | Approx. 75 lbs (34 kg) (Short shaft model) |
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2. Common Maintenance & Tune-up Specs
Keeping a vintage two-stroke outboard running smoothly requires regular maintenance of its fuel, ignition, and cooling systems. Below are the precise tune-up values and part classifications for the 1991 model year.
Ignition System Specs
- Spark Plug Type: NGK BP8H-N-10 (or Champion L78V / QL78V for suppression applications)
- Spark Plug Gap: 0.040 inches (1.0 mm)
- Spark Plug Torque: 18.5 lb-ft (25 N·m)
- Points Gap: Not Applicable. The 1991 model uses a solid-state, breakerless Capacitor Discharge Ignition (CDI) system. There are no points to gap or replace.
Cooling & Water Pump (Impeller) Information
The cooling system relies on a rubber impeller located in the lower unit’s water pump housing. Inspect this annually and replace it every two seasons or immediately if water flow from the “tell-tale” stream weakens.
- Replacement Impeller Part Number: Mercury OEM # 47-43026T2 (or equivalent Sierra 18-3056)
- Water Pump Kit (with housing & plates): OEM # 46-42089A5
- Cooling System Type: Thermostatically controlled, water-cooled bypass system.
Lower Unit Lubrication
Change the gearcase lubricant at the end of every season to prevent damage from potential water intrusion.
- Gear Lubricant Type: High-Performance Outboard Gear Lube (SAE 80W-90 / API GL-5)
- Gearcase Capacity: Approximately 6.8 fl. oz. (200 ml)
- Drain/Fill Plug Seals: Always replace the cardboard or nylon washers (OEM # 12-19183) when changing gear lube to prevent leaks.
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3. Notable Features & History
The Mariner brand was established by Brunswick Corporation in the mid-1970s, initially to target international markets outside the United States. Over time, it grew into a highly respected brand domestic to North America, known for rugged construction and simple, dependable engineering.
Design Integration with Mercury Marine
By 1991, Mariner engines in the 9.9 HP to 15 HP range were identical in internal design to Mercury outboards of the same year, manufactured under Brunswick’s unified production lines. While earlier, smaller Mariners were often manufactured in Japan by Yamaha or Tohatsu, the 1991 US-market Mariner 15 HP is a “Mercury-designed” platform. This is highly beneficial for current owners, as parts are widely available, affordable, and interchangeable with Mercury equivalents.
Key Design Features
- Tiller Handle Multi-Function Control: One of the standout ergonomic features of this era was the “all-in-one” tiller handle. It combined shifting (Forward, Neutral, Reverse), throttle control, steering friction adjustment, and the engine kill button into a single, intuitive handle design.
- Lightweight Portability: Weighing in at approximately 75 pounds, the 1991 Mariner 15 HP offered one of the best power-to-weight ratios in its class. This made it a favorite for boaters who needed to remove the motor from the transom for transport or storage.
- Loop Charged Induction: Loop charging directs the fuel/air mixture into the combustion chamber more precisely than older cross-flow designs. This results in cleaner combustion, better fuel economy, and crisp throttle response.
- Through-Hub Exhaust: Routing the exhaust gasses through the propeller hub keeps the engine remarkably quiet and helps discharge noxious fumes underwater, away from the boat’s occupants.