Yamaha EK25B two-stroke (kerosene) service guide
Oil, gear oil, spark plugs and the service schedule for the Yamaha EK25B two-stroke (kerosene) (25 hp, two-stroke), from the factory service manual, plus how we service it in Hawaiʻi.
Models covered: EK25BMH two-stroke twin, worldwide name only (no USA or Canada name is listed): 2 cylinders, 496 cm³ (30.3 cu in), 72.0 x 61.0 mm, one carburetor, manual start. A kerosene engine: main fuel kerosene, sub fuel unleaded regular gasoline, both premixed with oil. Approval model code 62C, starting serial S: 100101, L: 400101, Y: 750101.
Quick answers
- What two-stroke oil does a Yamaha EK25B two-stroke use?
- 2 stroke outboard engine oil, grade TC-W3.
- What is the fuel to oil ratio for a Yamaha EK25B two-stroke?
- Main fuel kerosene: mixing ratio 30:1. Sub fuel unleaded regular gasoline: mixing ratio 50:1.
- What gear oil does a Yamaha EK25B two-stroke use, and how much?
- Hypoid gear oil, SAE #90 (API GL-4). Quantity: 320 cm³ (10.8 US oz, 11.3 Imp oz); fill through the drain hole until oil flows out of the check hole with no air bubbles.
- What spark plug does a Yamaha EK25B two-stroke use?
- NGK B7HS-10, qty 2. Gap: 0.9 to 1.0 mm (0.035 to 0.039 in).
- How often should a Yamaha EK25B two-stroke be serviced?
- The factory schedule lists: Initial 10 hours; 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 EK25B two-stroke?
- The factory schedule gives no replacement interval. 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 EK25B two-stroke?
- 1,300 ± 50 r/min; pilot screw turns out: gasoline carburetor 1-1/2 ± 1/2, kerosene carburetor 1/2 (+2, -1/2).
- What is the propeller nut torque on a Yamaha EK25B two-stroke?
- 35 N·m (3.5 kgf·m, 26 ft·lb).
Maintenance specs
| Two-stroke engine oil | 2 stroke outboard engine oil, grade TC-W3 |
|---|---|
| Fuel to oil ratio | Main fuel kerosene: mixing ratio 30:1. Sub fuel unleaded regular gasoline: mixing ratio 50:1. |
| Gear oil | Hypoid gear oil, SAE #90 (API GL-4) |
| Gear oil quantity | 320 cm³ (10.8 US oz, 11.3 Imp oz); fill through the drain hole until oil flows out of the check hole with no air bubbles |
| Gear oil drain and check screws | Install the level check screw first, then quickly install the drain screw |
| Lower unit holding pressure | 100 kPa (1.0 kg/cm², 14.5 psi) for 10 seconds at the check hole; do not overpressurize. Leakage tester 90890-06762 |
| Spark plug | NGK B7HS-10, qty 2 |
| Spark plug gap | 0.9 to 1.0 mm (0.035 to 0.039 in) |
| Spark plug torque (M14) | 25 N·m (2.5 kgf·m, 18 ft·lb); without a torque wrench, 1/4 to 1/2 turn past finger tight |
| Idle speed | 1,300 ± 50 r/min; pilot screw turns out: gasoline carburetor 1-1/2 ± 1/2, kerosene carburetor 1/2 (+2, -1/2) |
| Thermostat | Opens at 48 to 52 °C (118 to 126 °F); fully open at 60 °C (140 °F); minimum valve lift 3 mm (0.12 in) |
| Propeller nut | 35 N·m (3.5 kgf·m, 26 ft·lb) |
| Power trim and tilt | None: manual tilt, tilt 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
Replace: gear case oil, gear case plug gaskets
Check: spark plugs, fuel filter, anodes
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, 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 EK25BMH service manual, 69T-28197-ZA-11. Specs can change by model year and serial number, so always confirm your exact model from the serial tag on the clamp bracket.