Yamaha 20C two-stroke service guide
Oil, gear oil, spark plugs and the service schedule for the Yamaha 20C two-stroke (20 hp, two-stroke), from the factory service manual, plus how we service it in Hawaiʻi.
Models covered: 20CMH two-stroke twin, tiller handle, manual start: 2 cylinders, 430.0 cm³ (26.24 cu in), 67.0 x 61.0 mm, premixed fuel and oil 50:1. Starting serial number S: 1001031
Quick answers
- What two-stroke oil does a Yamaha 20C two-stroke use?
- 2-stroke outboard motor oil, NMMA-certified TC-W3.
- What is the fuel to oil ratio for a Yamaha 20C two-stroke?
- Pre-mixed fuel and oil 50:1 (regular gasoline).
- What gear oil does a Yamaha 20C two-stroke use, and how much?
- Hypoid gear oil, API GL-4, SAE 90. Quantity: 0.18 L = 180 cm³ (6.09 US oz, 6.07 Imp oz); fill through the oil-drain hole until it overflows from the oil-level hole.
- What spark plug does a Yamaha 20C two-stroke use?
- NGK B7HS (BR7HS is the factory option for China), qty 2. Gap: 0.6 to 0.7 mm (0.024 to 0.028 in).
- How often should a Yamaha 20C two-stroke be serviced?
- The factory schedule lists: Initial 10 hours (1 month); Initial 50 hours (3 months); Every 100 hours (6 months); Every 200 hours (1 year). In Hawaiʻi's salt water we do a full yearly service every 200 hours or 1 year, whichever comes first, and a new engine needs its first (break-in) service.
- When should the water pump impeller be replaced on a Yamaha 20C two-stroke?
- The factory schedule says to inspect it at 200 hours. In Hawaiʻi's warm salt water we replace it every 200 hours or 1 year, whichever comes first, because a worn impeller is the most common cause of overheating.
- What is the idle speed of a Yamaha 20C two-stroke?
- 950 to 1,050 r/min; pilot screw 1 1/8 to 1 5/8 turns out.
- What is the propeller nut torque on a Yamaha 20C two-stroke?
- 30 N·m (3.0 kgf·m, 22 ft·lb), then fit the cotter pin.
Maintenance specs
| Two-stroke engine oil | 2-stroke outboard motor oil, NMMA-certified TC-W3 |
|---|---|
| Fuel to oil ratio | Pre-mixed fuel and oil 50:1 (regular gasoline). |
| Gear oil | Hypoid gear oil, API GL-4, SAE 90 |
| Gear oil quantity | 0.18 L = 180 cm³ (6.09 US oz, 6.07 Imp oz); fill through the oil-drain hole until it overflows from the oil-level hole |
| Gear oil drain and level plugs | Refit the oil-level plug first, then the drain plug |
| Lower unit holding pressure | 100 kPa (15 psi) at the oil-level hole with the drain screw tight, must hold 10 seconds |
| Spark plug | NGK B7HS (BR7HS is the factory option for China), qty 2 |
| Spark plug gap | 0.6 to 0.7 mm (0.024 to 0.028 in) |
| Spark plug torque | 28 N·m (2.8 kgf·m, 20 ft·lb) |
| Idle speed | 950 to 1,050 r/min; pilot screw 1 1/8 to 1 5/8 turns out |
| Thermostat | Opens at 58.0 to 62.0 °C (136.4 to 143.6 °F); fully open at 70.0 °C (158.0 °F); valve open lower limit 3.0 mm (0.12 in) |
| Anodes | External anode on the gear case (align the marks made at removal); Clean with a wire brush, replace if worn out, never paint. |
| Propeller nut | 30 N·m (3.0 kgf·m, 22 ft·lb), then fit the cotter pin |
| Power trim and tilt | None: manual tilt, trim angles 4, 8, 12, 16, 20 degrees |
Service schedule
The factory maintenance intervals, with what is replaced and what is checked at each. Rows marked Hawaiʻi are our shop's yearly service for salt water use.
Initial 10 hours (1 month)
Replace: gear case oil, gear case plug gaskets
Check: spark plugs, fuel filter
Initial 50 hours (3 months)
Replace: gear case plug gaskets, propeller cotter pin
Check: spark plugs, fuel filter, anodes
Every 100 hours (6 months)
Replace: gear case oil, gear case plug gaskets, propeller cotter pin
Check: spark plugs, fuel filter, anodes
Every 200 hours (1 year)
Replace: gear case oil, gear case plug gaskets, propeller cotter pin, water pump impeller
Check: spark plugs, fuel filter, thermostat, anodes
We replace the water pump impeller at every service from 200 hours up, which can be sooner than the factory schedule, because of Hawaiʻi’s warm salt water.
Source: Yamaha 20C service manual. Specs can change by model year and serial number, so always confirm your exact model from the serial tag on the clamp bracket.