1975 Evinrude 15 HP Fastwin Outboard Specs & Part Numbers
Technical Guide: 1975 Evinrude 15 HP Fastwin Outboard Motor
The 1975 Evinrude 15 HP Fastwin (Model Series 15504 / 15505) is one of the most celebrated portable outboard motors in marine history. Introduced during a golden era of OMC (Outboard Marine Corporation) engineering, this motor belongs to the legendary 9.9 HP and 15 HP family first launched in 1974. Known for its lightweight frame, exceptional power-to-weight ratio, and relentless reliability, it remains a favorite among vintage outboard collectors, anglers, and DIY mechanics today.
This technical guide provides a comprehensive overview of the specifications, maintenance requirements, and historical significance of this classic two-stroke outboard.
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1. Overview & Technical Specifications
The 1975 Evinrude 15 HP utilizes a two-cylinder, loop-charged, two-stroke powerhead. It was designed to replace the older, low-profile 9.5 HP “turtleback” design, offering a massive jump in mid-range torque and top-end speed while keeping the overall package light enough for easy manual carrying and transom mounting.
| Specification | Value / Measurement |
|---|---|
| Horsepower | 15 HP @ 5500 RPM |
| Engine Type | 2-Cylinder, 2-Stroke, Cross-Flow Scavenged |
| Displacement | 13.2 cubic inches (216 cc) |
| Bore & Stroke | 2.250″ x 1.656″ (57.15 mm x 42.06 mm) |
| Fuel/Oil Ratio | 50:1 (Regular Unleaded 87 Octane to TC-W3 Marine Oil) |
| Starting System | Manual Recoil (Electric start options were available under model 15505) |
| Weight | Approx. 68 lbs (31 kg) for short shaft manual start |
| Transom Height | 15-inch (Short Shaft) or 20-inch (Long Shaft) |
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2. Common Maintenance & Tune-up Specs
Keeping a 1975 Fastwin running optimally requires keeping up with basic ignition, fuel system, and cooling system maintenance. Because this motor was built before the widespread adoption of electronic ignition (CDI) on this specific platform, servicing the points and condensers under the flywheel is a critical component of annual tune-ups.
Ignition System
- Spark Plugs: Champion L77JC4 (or QL77JC4 for suppression of radio interference).
- Spark Plug Gap: 0.030 inches (0.76 mm).
- Ignition System Type: Magneto ignition with dual points and external coils.
- Breaker Point Gap: 0.020 inches (0.51 mm) at high point of the cam lobe.
Fuel System
- Carburetor: Single-barrel, float-feed with manual choke and adjustable low-speed idle mixture needle.
- High-Speed Jet: Fixed jet (Orifice size varies slightly by altitude; stock is typically #54 or #56).
- Fuel Mix Formula: 50:1 ratio. This translates to 1 pint (16 oz) of high-quality TC-W3 2-cycle oil to 6 gallons of regular unleaded fuel. Avoid using ethanol fuels if possible, as ethanol breaks down vintage rubber fuel lines and gum up the carburetor float bowl.
Cooling & Gearcase
- Impeller / Water Pump Part Number: OEM #386084 (or equivalent Sierra 18-3050). The water pump housing and impeller are located on top of the lower unit gearcase. It should be replaced every 2 to 3 seasons, or immediately if the motor has been run dry.
- Gearcase Lubricant: OMC Hi-Vis Marine Gear Lube (or equivalent API GL-5 80W-90 marine gear oil).
- Gearcase Capacity: Approximately 9 fluid ounces. Fill from the bottom drain hole until fluid exits the top vent hole to prevent air pockets.
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3. Notable Features & History
The 1975 Evinrude 15 HP Fastwin holds a special place in the lineage of portable outboards. To understand its value, one must look at the transition OMC made in the mid-1970s.
The Leap from the 9.5 HP “Low Profile”
From 1964 to 1973, OMC produced a popular 9.5 HP outboard that was highly compact but suffered from exhaust-smoke ingestion and limited power. In 1974, OMC scrapped that design entirely and engineered a brand-new 9.9 HP and 15 HP block. The 1975 15 HP Fastwin benefited directly from this redesign, offering a true 15-horsepower rating at the crankshaft with a much stiffer, more durable powerhead casting.
Key Engineering Features
- Tiller Throttle Twist Grip: Unlike older outboards where the throttle was operated by a dash lever, the 1975 Fastwin integrated the throttle control directly into the tiller handle, complete with a kill button on the end of the handle. This made one-handed operation seamless.
- Shifter Placement: A robust, front-mounted shift lever (Forward-Neutral-Reverse) eliminated awkward reaching to the side or back of the cowl, greatly increasing safety during tight docking maneuvers.
- The Pre-CDI Point System: While later models (1977 onwards) adopted solid-state Capacitive Discharge Ignition (CDI), the 1975 model relies on dual ignition points beneath the flywheel. While points require periodic adjustment and cleaning, they are incredibly robust and can be serviced in the field with basic hand tools, making this motor highly valued by survivalists and remote wilderness boaters.
- Carburetor and Exhaust Tuning: The 15 HP model shares the exact same block and displacement (13.2 cubic inches) as its 9.9 HP sibling. The extra 5 horsepower of the Fastwin 15 was achieved via a larger carburetor throat, a different reed valve block shim configuration, and slight changes to the exhaust tuner.
Legacy & Part Availability
Because OMC produced millions of these 9.9/15 HP outboards between 1974 and 2007 with minimal structural changes, parts availability is virtually unmatched. Almost every gasket, impeller, seal, ignition coil, and carburetor rebuild kit is still actively manufactured today by both OEM (BRP) and aftermarket manufacturers like Sierra Marine. For the modern enthusiast, this makes the 1975 Evinrude 15 HP Fastwin one of the easiest, most rewarding vintage outboards to restore and maintain.