1973 Evinrude 18 HP Fastwin Outboard Specs & Part Numbers
Introduction to the 1973 Evinrude 18 HP Fastwin
The 1973 Evinrude 18 HP Fastwin (Model series 18304 for rope start and 18305 for electric start) represents the pinnacle of OMC’s (Outboard Marine Corporation) classic two-stroke design era. Renowned for its rugged dependability, relative simplicity, and excellent power-to-weight ratio, this motor remains a favorite among vintage outboard enthusiasts and utility boaters alike. Whether you are restoring a classic aluminum fishing boat or maintaining a reliable workhorse, this guide provides the vital technical specifications and maintenance protocols required to keep this vintage machine running flawlessly.
Overview & Technical Specifications
The Fastwin 18 HP utilizes a twin-cylinder, loop-scavenged, two-stroke powerhead. By 1973, this basic engine platform had been refined over nearly two decades, making it one of the most mechanically reliable outboards ever built. It features a single-carburetor setup, a thermo-syphon cooling system regulated by a thermostat, and a standard shear-pin or thru-hub style prop drive depending on the exact lower unit configuration.
| Specification Parameter | Technical Value / Detail |
|---|---|
| Horsepower Rated | 18 HP @ 4,500 RPM |
| Cylinder Configuration | Inline 2-Cylinder (2-Stroke) |
| Displacement | 22.0 cubic inches (360.5 cc) |
| Bore & Stroke | 2.500″ x 2.250″ (63.5 mm x 57.15 mm) |
| Fuel/Oil Ratio | 50:1 (Regular 87 Octane Gas to TC-W3 2-Stroke Marine Oil) |
| Fuel Tank Style | Single-line, vacuum-pulse fuel pump system (Modern style) |
| Weight (Dry) | Approx. 79 lbs (35.8 kg) for manual start model |
Common Maintenance & Tune-up Specs
To keep the 1973 Fastwin running at peak efficiency, regular maintenance of the ignition system, fuel delivery network, and cooling system is required. Below are the critical tune-up tolerances, part numbers, and fluid capacities.
1. Ignition System
The 1973 model utilizes the classic OMC Universal Magneto system located beneath the flywheel. It relies on dual sets of breaker points, condensers, and ignition coils housed entirely under the flywheel.
- Spark Plug Type: Champion L77JC4 (or Champion L82C / UL81C depending on operating temperature preferences)
- Spark Plug Gap: 0.030 inches (0.76 mm)
- Breaker Point Gap: 0.020 inches (0.51 mm)
- Timing: Set points using the timing marks on the flywheel and armature plate, or use a timing fixture to ensure they break exactly at the correct crankshaft index.
2. Fuel System & Carburetion
The carburetor features an adjustable low-speed idle mixture needle and a fixed high-speed jet orifice. Do not attempt to drill or modify the fixed high-speed jet unless operating at extreme altitudes (above 5,000 feet).
- Initial Low-Speed Needle Adjustment: Gently seat the low-speed needle valve clockwise until it stops, then back it out 1.5 turns counter-clockwise. Adjust in 1/8-turn increments with the motor running in gear at operating temperature to achieve a smooth idle.
- Fuel Pump: Vacuum-pulse operated, square metal-bodied fuel pump (rebuildable with common OMC fuel pump kits).
3. Cooling System (Water Pump & Impeller)
The water pump is located inside the lower gearcase housing. It must be serviced regularly to prevent overheating, which can quickly warp or score the cylinder walls.
- Impeller Part Number: OEM #379764 (Sierra equivalent: 18-3001)
- Thermostat Specification: Opens at 143°F (61°C). The thermostat housing is situated on the cylinder head cover.
- Service Interval: Inspect and/or replace the impeller every 2 years or 100 running hours (more frequently if operated in sandy or silty water).
4. Gearcase (Lower Unit) Lubrication
The lower unit uses a robust mechanical shift dog system. It requires clean, moisture-free gear lubricant to prevent corrosion and premature gear wear.
- Lubricant Type: OMC Premium Blend (Type C) or high-quality Marine SAE 80W-90 Gear Lube.
- Gearcase Capacity: Approximately 9 fluid ounces (266 ml).
- Drain/Fill Seals: Always replace the nylon gaskets on the “Drain” and “Fill” plug screws during every oil change to prevent water intrusion.
Notable Features & History
The Final Era of the Classic 18 HP
The 18 HP Fastwin has a storied history in the Outboard Marine Corporation lineup. First introduced in the 1950s as a 15 HP model, it was upgraded to 18 HP in 1957. By 1973, this design had reached its absolute refinement. 1973 was one of the final years of production for the 18 HP configuration; OMC phased out the 18 HP shortly thereafter in favor of the newly designed 9.9/15 HP platform and revamped 20/25 HP blocks.
Transition to Modern Conveniences
Unlike early 1950s models, the 1973 Fastwin features a modern, single-line fuel system powered by a crankcase pulse-driven fuel pump. This eliminated the dangerous, pressurized dual-line fuel tanks that characterized mid-century outboards. Owners can safely use standard, modern portable marine fuel tanks with standard OMC quick-disconnect fittings.
Bulletproof “OMC Universal Magneto”
While later outboards transitioned to solid-state CDI (Capacitor Discharge Ignition) electronics, the 1973 model stuck with the legendary, mechanically simple under-flywheel magneto. Though points and condensers require occasional adjustment and replacement, they are highly reliable, immune to voltage spikes that destroy modern CDI packs, and can be easily repaired with basic hand tools anywhere in the world.
The “Sister” Motor
During this era of badge engineering, Evinrude and Johnson marketed identical outboards under different names. The Johnson equivalent of this motor is the 1973 Johnson 18 HP Sea-Horse. Aside from cosmetic paint schemes (Evinrude’s traditional light blue vs. Johnson’s seafoam green/white) and minor cowling shapes, the physical powerheads, gearcases, and internal components are completely interchangeable.