1973 Mercury 20 HP Merc 200 Outboard Specs & Part Numbers
1973 Mercury 20 HP (Merc 200) Technical Guide
The 1973 Mercury 20 HP, commercially designated as the Merc 200, is a legendary twin-cylinder two-stroke outboard motor. Known for its distinct “Phantom Black” styling, lightweight construction, and impressive power-to-weight ratio, this motor remains a favorite among vintage outboard enthusiasts, anglers, and DIY restorers. This guide provides comprehensive technical specifications, maintenance procedures, and historical context for maintaining or restoring this classic powerplant.
—
Overview & Specifications
The Merc 200 is an inline 2-cylinder outboard featuring a reed-valve induction system and loop-scavenged design. In 1973, Mercury designed these motors to be highly portable yet powerful enough to plane small utility boats, jon boats, and classic runabouts.
| Specification | Value / Detail |
|---|---|
| Propshaft Horsepower | 20 HP @ 5000 – 5500 RPM |
| Configuration | Inline 2-Cylinder, 2-Stroke |
| Displacement | 22.0 Cubic Inches (360.5 cc) |
| Bore & Stroke | 2.562″ x 2.125″ (65.07 mm x 53.97 mm) |
| Fuel/Oil Ratio | 50:1 (87 Octane Unleaded + TC-W3 Marine 2-Stroke Oil) |
| Cooling System | Water-cooled (Shear-pin or Spline-driven Impeller) |
| Weight | Approx. 82 lbs (37.2 kg) – Manual Start, Short Shaft |
| Ignition System | Thunderbolt Phase Maker Ignition (or Maker Point Ignition) |
—
Common Maintenance & Tune-up Specs
To keep the 1973 Merc 200 running reliably, routine ignition, fuel system, and lower unit servicing are essential. Below are the critical tune-up tolerances and part profiles required for maintenance.
1. Ignition & Spark Specs
- Spark Plug Type:
- Surface Gap (Standard): NGK BUHW-2 or Champion L77V. These plugs do not require gapping.
- Standard Electrode (Alternative): Champion L78C (Gap set to 0.030″ / 0.76 mm).
- Point Gap (Phase Maker System): Set points to 0.018″ to 0.020″ (0.45 mm to 0.50 mm). Note that the “Phase Maker” design breaks the circuit to initiate spark; precise cleaning and alignment of the point faces are critical for consistent ignition performance.
- Timing Specs: Timing is set dynamically with a timing light or statically with a dial indicator. Full advance timing is typically .341″ (8.66 mm) Before Top Dead Center (BTDC) at wide-open throttle (WOT).
2. Carburetor & Fuel Delivery
- Carburetor Type: Tillotson (typically BC or KB series) or Walbro integral pump carburetor.
- Idle Mixture Screw Adjustment: Start by gently seating the idle needle valve clockwise until bottomed out (do not force it), then back it out 1 to 1-1/4 turns. Fine-tune in the water under load while idling in forward gear.
- Fuel Lines: Replace any legacy rubber lines with modern 5/16″ alcohol-resistant marine fuel lines to prevent degradation from modern ethanol-blend fuels.
3. Lower Unit & Cooling System
- Gearcase Lubricant: Use high-quality marine SAE 90 Outboard Gear Lubricant (Mercury Premium Gear Lube).
- Capacity: Approx. 7.5 fl. oz. (221 ml). Fill from the bottom drain hole until fluid exits the top vent hole.
- Water Pump Impeller: OEM Part Number: 47-89981 / 47-89981T1 (Sierra replacement: 18-3017).
- Service Interval: Inspect annually; replace every 2 years or sooner if run in sandy/silty waters.
—
Notable Features & History
“The Merc 200 represents the pinnacle of Mercury’s early-1970s lightweight utility design, bridging the gap between portable fishing motors and heavier multi-cylinder engines.”
The “Phase Maker” Ignition Transition
In the early 1970s, Mercury transitioned away from traditional magneto ignitions toward solid-state and capacitor discharge systems. The 1973 Merc 200 features a unique ignition hybrid system often referred to as the “Phase Maker” ignition. This system used a low-tension maker-point configuration to charge an external ignition coil. While highly reliable when properly adjusted, it requires clean contacts; even a tiny film of oil on the points can interrupt the circuit and prevent the engine from starting.
Unmatched Portability and Ergonomics
Weighing in at just over 80 pounds, the 1973 Merc 200 offered an outstanding power-to-weight ratio. Competing manufacturers’ 20 HP offerings of the era were often bulky and weighed 10 to 15 pounds more. Mercury achieved this through a compact inline block configuration and cast-aluminum components. The “wraparound” cowl design allowed easy access to the powerhead by simply flipping two latches and lifting off the lightweight aluminum shroud.
Legacy and Collectibility
Today, the 1973 Mercury 20 HP is highly prized by collectors and vintage boaters. Thanks to its robust build quality, parts are still widely available through aftermarket channels like Sierra Marine and vintage outboard networks. When properly tuned with fresh gear lube, clean points, and non-ethanol 50:1 fuel, these motors are incredibly reliable, offering the signature classic Mercury two-stroke exhaust note and impressive top-end performance.