1978 Mercury 4.5 HP Merc 45 Outboard Specs & Part Numbers
Introduction
The 1978 Mercury 4.5 HP outboard motor, often referred to as the Merc 45, is a legendary single-cylinder, two-stroke marine engine. Renowned for its lightweight design, reliability, and robust construction, this motor has remained a favorite among vintage outboard collectors, anglers, and dinghy owners. This technical guide provides comprehensive specifications, maintenance data, and historical context for servicing and restoring this classic powerplant.
Overview & Specifications
The Merc 45 is a compact, loop-charged single-cylinder engine. It represents the pinnacle of Mercury’s portable utility line of the late 1970s, balancing portability with the dependable power needed for small fishing boats, canoes, and auxiliary sailboat propulsion.
| Specification | Technical Data |
|---|---|
| Horsepower | 4.5 HP @ 5000 RPM |
| Cylinder Configuration | Single (1) Cylinder, 2-Stroke |
| Displacement | 6.7 cubic inches / 109.8 cc |
| Bore & Stroke | 2.125 in x 1.875 in (53.97 mm x 47.63 mm) |
| Fuel-to-Oil Mixture Ratio | 50:1 (Unleaded gasoline to TC-W3 2-cycle outboard oil) |
| Fuel Delivery | Single Tillotson or Walbro carburetor with integral fuel pump |
| Ignition System | “Phase Maker” Point Ignition or Solid-State CDI (depending on Serial Number) |
| Starting System | Manual Recoil (Pull-start) |
| Dry Weight | Approximately 36 lbs (16.3 kg) |
Common Maintenance & Tune-up Specs
Keeping a vintage 1978 Merc 45 running smoothly requires attention to ignition settings, fuel mixture, and the cooling system. Below are the precise technical specifications needed for a standard tune-up.
Ignition & Electrical
- Spark Plug Type: NGK BP6HS or Champion L86C (Ensure standard 1/2-inch reach)
- Spark Plug Gap: 0.040 inches (1.0 mm)
- Breaker Point Gap (If equipped with Point Ignition): 0.020 inches (0.51 mm)
- Timing: Set to factory marks on the flywheel bracket (typically 0.010″ to 0.012″ Before Top Dead Center at idle check, though dynamic timing is fixed via the mechanical linkage).
Carburetor Tuning
- Initial Idle Mixture Screw Setting: Gently seat the needle, then turn it counter-clockwise (out) 1 to 1-1/4 turns. Fine-tune while the engine is warm and running in gear.
- Float Height: Parallel to the bowl gasket surface when inverted.
Cooling & Lower Unit
- Water Pump Impeller Part Number: Mercury OEM 47-89981 / 47-89981Q1 (or aftermarket Sierra 18-3007).
Note: Always replace the impeller key and wear plate when servicing the water pump.
- Gearcase Lubricant Type: High-Quality Marine SAE 80W-90 Gear Lube (API GL-5 rated, formulated for outboard gearcases).
- Gearcase Capacity: Approximately 3.0 fl. oz. (90 ml). Fill from the bottom drain hole until fluid emerges from the upper vent hole to prevent air pockets.
- Propeller Drive: Shear pin drive setup. Always carry spare brass shear pins and cotter keys on board.
Notable Features & History
The 1978 Merc 45 represents a critical transition era in marine engineering. During the mid-to-late 1970s, outboard manufacturers were under pressure to improve emissions and fuel efficiency while transitioning away from older breaker-point ignition systems to electronic solid-state designs.
The “Phase Maker” Ignition Legacy
Depending on the exact serial number range of your 1978 model, it may feature Mercury’s unique “Phase Maker” ignition system. Developed as an intermediate step between traditional magneto points and modern CDI (Capacitor Discharge Ignition), the Phase Maker system used breaker points to ground and interrupt the primary ignition circuit to trigger a rapid voltage spike. While highly effective when new, keeping these points clean and properly gapped is essential for easy starting today.
All-Metal Construction
Unlike modern outboards that rely heavily on composite plastics for cowlings and brackets, the 1978 Merc 45 is constructed almost entirely of aluminum alloys and stainless steel. It features Mercury’s famous “Phantom Black” paint scheme with distinctive red and silver wrap-around decals. This rugged build quality means that with basic maintenance, these motors are highly resilient against corrosion and physical wear.
Loop Charging vs. Cross-Flow
The Merc 45 utilized a loop-charged combustion chamber design. Instead of using a deflector piston (common in older cross-flow engines), loop-charged engines use precisely angled ports to direct the incoming fuel/air mixture up into the cylinder dome. This results in more complete combustion, better fuel economy, and higher power output per cubic inch of displacement.
Tips for Modern Operation
Because this engine was engineered before the widespread use of ethanol-blended fuels, owners should strictly use ethanol-free gasoline (rec 90). Ethanol can degrade the vintage rubber fuel lines and internal carburetor diaphragms. Always use high-quality TC-W3 designated oil to ensure the internal needle and sleeve bearings remain lubricated during high-RPM operation.