Johnson 15 HP 20″ Long Shaft (1974) Outboard Motor Specifications & Maintenance Guide
The 1974 Johnson 15 HP Model 15RL74 is a long-shaft, manual-start two-stroke outboard engineered for deeper transoms, commercial skiffs, and sailboat auxiliary power. Featuring a 20-inch shaft extension, a dry weight of 66 pounds, and a manual self-winding pull starter, Model 15RL74 provides excellent maneuverability and dependable 15-horsepower output across demanding marine applications.
To preserve smooth idling and dependable throttle response across all operating ranges, regular service according to OEM specifications is essential. Maintaining exact tune-up tolerances—such as gapping Champion UL4J spark plugs to 0.030 inches, torquing plugs to 17-1/2–20-1/2 ft-lbs, setting breaker points to 0.020 inches, and checking condenser capacity (0.25–0.29 Mfd)—directly protects the low-tension magneto system and powerhead from unexpected failure.
Maintenance Recap & Tune-Up Checklist
- Ignition System Tuning: Set Champion UL4J (14mm) spark plug gaps to 0.030 inches, torque plugs to factory spec (17-1/2–20-1/2 ft-lbs), and inspect breaker point gaps to maintain a strict 0.020-inch opening.
- Magneto & Electrical Maintenance: Inspect ignition coil (Part No. 581407) insulation, verify condenser (Part No. 580422) reads between 0.25 and 0.29 Mfd, and inspect the 5-amp flywheel alternator connections.
- Fuel & Gearshift Inspection: Check fuel delivery from the 6-gallon remote fuel tank, inspect manual choke linkage, and verify smooth shift linkage operation through Forward, Neutral, and Reverse positions.
- Shaft & Lower Unit Service: Check lower unit gear lubricant levels on the 20-inch long-shaft assembly, inspect the water pump impeller for positive cooling discharge, and inspect propeller hardware.
Technical Specifications
| Specification Parameter | Details / Value |
|---|---|
| Model Number | 15RL74 |
| Model Year | 1974 |
| Horsepower Rating | 15 HP |
| Outboard Weight (Without tank) | 66 lbs. |
| Transom Length | 20″ (Long Shaft) |
| Starter Type | Manual – Self-winding / rope |
| Gear Shift Control | Forward, neutral and reverse |
| Fuel Capacity | 6 gallons |
| Ignition System Type | Low tension magneto |
| Spark Plug Type | Champion UL4J (14mm) |
| Spark Plug Gap | .030 inch |
| Spark Plug Torque | 17-1/2 – 20-1/2 foot-pounds |
| Breaker Point Gap | .020 inch |
| Condenser Part Number | Part Number 580422 |
| Condenser Capacity | .25 – .29 Mfd. |
| Coil Part Number | Part No. 581407 |
| Electrical System | 5 amp flywheel alternator |
| Starter Amperage Draw (Cranking) | N/A |
Disclaimer & Information Notice: Specifications, fuel ratios, and maintenance guidelines listed on VintageMotorSpecs.com are compiled for historical reference and educational purposes only. Always verify data against an official OEM service manual prior to ordering parts or executing repairs. VintageMotorSpecs.com assumes no liability for repair errors, equipment damage, or personal injury resulting from the use of this data.
Spot an error or discrepancy in these specs? Please contact us with your engine details so we can update our database.
Non-Affiliation Notice: VintageMotorSpecs.com is an independent historical reference database and is not affiliated, associated, authorized, endorsed by, or in any way officially connected with Outboard Marine Corporation (OMC), Mercury Marine, Yamaha, Honda, or any of their subsidiaries or affiliates. All product names, trademarks, and registered trademarks are property of their respective owners.