Yamaha 1984-91 70 HP Outboard Specs & Maintenance
Looking to keep your classic 1984–1991 70 HP 3-cylinder 2-stroke outboard running at peak performance? Built on a durable 52 cubic inch (849 cc) inline block, this legendary engine offers incredible power-to-weight performance for mid-sized vintage boats. Whether you are troubleshooting a misfire or performing a routine tune-up, our guide provides the exact specs and service advice DIY marine mechanics need to keep this workhorse pushing water.
Complete Technical Specifications
| Specification | Details |
|---|---|
| Model / Horsepower | 70 |
| Year Range | 1984-91 |
| Engine Type | IL 2-stroke |
| Cylinders | 3 |
| Displacement | 52 cu-in (849 cc) |
| Bore x Stroke | 2.83 x 2.74 in (72.0 x 69.5 mm) |
| Compression Ratio | 6.2:1 |
| Full Throttle RPM | 4500-5500 |
| Idle Speed (Neutral) RPM | 750-850 |
| Idle Speed (Trolling) RPM | 550-650 |
| Spark Plug (NGK) | B8HS-10 |
| Spark Plug (Champion) | L78C |
| Spark Plug Gap | 0.035-0.039 in (0.9-1.0 mm) |
| Ignition System | CDI Micro |
| Ignition Timing (Idle) | 6-8 ATDC |
| Ignition Timing (Carb Pickup) | 6-8 ATDC |
| Ignition Timing (Full Advance) | 19-21 |
| Fuel System | Carb |
| Starting System | Electric |
| Oil Injection System | PB or 50:1 PM |
| Injection Oil Ratio | PB or 50:1 PM |
| Gear Oil Capacity (mL/cc) | 610 |
| Gear Oil Capacity (Oz) | 20.6 |
| Approximate Weight | 203-247 lbs (92-112 kg) |
| Battery CCA / Reserve | 380 (70) |
| Battery Reserve | 124 minutes |
| Tach Pole Setting | 6 |
| Test Propeller Part Number | YB-1620 |
| Test Propeller New Engine RPM | 4950-5150 |
Maintenance Summary
Routine maintenance is critical for this 52 ci 3-cylinder two-stroke. Keep the ignition system performing properly by installing NGK B8HS-10 spark plugs, gapped precisely between 0.035 and 0.039 inches (0.9–1.0 mm). Always inspect gear lube, check fuel filters, and service the water pump impeller annually.
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Notice: Specifications, fuel ratios, and maintenance guidelines listed on VintageMotorSpecs.com are compiled for historical reference and educational purposes only.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.