1977 Mercury 9.8 HP Merc 110 Outboard Specs & Part Numbers
The 1977 Mercury 9.8 HP (commonly referred to as the Merc 110) is a legendary twin-cylinder two-stroke outboard motor. Highly regarded for its excellent power-to-weight ratio, durability, and classic “Phantom Black” styling, this motor remains a favorite among vintage boat enthusiasts, anglers, and DIY mechanics.
This technical guide provides comprehensive specifications, maintenance data, and historical context to help you identify, service, and maintain this classic outboard.
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1. Overview & Technical Specifications
The Merc 110 is a loop-charged, two-cylinder motor designed for light utility boats, Jon boats, and small tenders. Below are the primary engineering and operational specifications for the 1977 model year:
| Specification | Detail / Value |
|---|---|
| Manufacturer & Model | Mercury Marine “Merc 110” |
| Year of Manufacture | 1977 |
| Horsepower Rating | 9.8 HP @ 5000–5500 RPM |
| Cylinder Configuration | 2-Cylinder (In-line, Two-Stroke) |
| Displacement | 10.9 cubic inches (approx. 179 cc) |
| Bore & Stroke | 2.000 in x 1.750 in (50.8 mm x 44.45 mm) |
| Fuel-to-Oil Ratio | 50:1 (Regular Unleaded / TC-W3 2-Cycle Outboard Oil) |
| Cooling System | Water-cooled (Shear-pin driven impeller) |
| Weight | ~65 lbs (29.5 kg) depending on shaft length |
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2. Common Maintenance & Tune-Up Specs
To keep the 1977 Merc 110 running smoothly, routine ignition, fuel, and cooling system maintenance is essential. Below are the precise settings and part specifications required for standard tune-ups:
Ignition System
- Ignition Type: Thunderbolt Phase Maker Ignition. Depending on the exact serial number range, some 1977 units utilized breaker points under the flywheel, while others had transitioned to solid-state capacitor discharge (CD) systems.
- Points Gap (if equipped with maker points): 0.020 inches (0.50 mm).
- Spark Plug Type:
- Standard Option: Champion L78V (Surface gap plug, no gapping required).
- Electrode Option (Alternative): NGK B7HS-10 or Champion L82C.
- Spark Plug Gap (for electrode plugs): 0.030 to 0.040 inches (0.76 mm – 1.01 mm). Standard recommendation is typically 0.035″.
Fuel & Carburetor Adjustment
- Carburetor Model: Tillotson (typically WC or KB series) or Walbro integral pump carburetor.
- Low-Speed Idle Mixture Screw: Start at 1-1/4 turns open from a lightly seated position. Adjust while warm and in forward gear for optimal idle.
- Float Height Setting: Parallel with the gasket surface when the carburetor body is inverted (approx. 13/32 in).
Cooling & Lower Unit Gearcase
- Water Pump Impeller: Mercury OEM Part Number 47-89981-1 (or Sierra aftermarket equivalent 18-3017).
Note: It is highly recommended to inspect and replace the impeller every 1–2 seasons. Operating this engine with a degraded impeller will lead to rapid overheating and powerhead failure.
- Gearcase Lubricant: High-quality SAE 90 Outboard Gear Lube (Premium Blend).
- Gearcase Oil Capacity: Approximately 6.0 fl. oz. (177 ml). Fill from the bottom drain hole until fluid exits the top vent hole to prevent air pockets.
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3. Notable Features, Design & History
The “Merc 110” moniker was introduced by Kiekhaefer Mercury in the early 1960s to represent their 9.8 HP motor. By 1977, the design had been refined to offer incredible reliability, making it one of the most successful portable outboards in history.
Key Design Features:
- “Phantom Black” Styling: By 1977, Mercury had fully embraced the sleek, all-black aesthetic. The cowl featured iconic red, blue, and silver horizontal decal wraps, giving the engine a modern, aggressive look.
- Integrated Twist-Grip Throttle: Unlike competitor outboards of the era that still relied heavily on cowl-mounted levers, the Merc 110 featured a highly responsive tiller-mounted twist-grip throttle, vastly improving ergonomics and control.
- Lightweight Portability: Weighing under 70 pounds, this engine could easily be transported in the trunk of a car and mounted by a single person, making it the premier choice for weekend fisherman.
- Thru-Hub Exhaust: Mercury’s inclusion of thru-hub exhaust reduced operating noise and directed exhaust gases underwater, which improved idle quality and minimized smoke around the transom.
Historical Significance & Legacy
In the mid-1970s, competitive dynamics between Mercury and OMC (Evinfude/Johnson) were fierce. While OMC offered a 9.9 HP motor that was physically larger and heavier, Mercury’s 9.8 HP Merc 110 was prized for its compact footprint and high RPM performance.
For DIY mechanics today, the 1977 Merc 110 represents the pinnacle of “simple” outboard technology before the widespread implementation of complex electronics and emission controls. With basic hand tools, an original service manual, and a steady supply of readily available vintage parts, these motors can easily be kept running for decades to come.