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Tohatsu MFS 8 / 9.8 carbureted (A3 and B) service guide

Oil, gear oil, spark plugs and the service schedule for the Tohatsu MFS 8 / 9.8 carbureted (A3 and B) (8 to 9.8 hp, four-stroke), from the factory service manual, plus how we service it in Hawaiʻi.

Models covered: MFS 8B and 9.8B and MFS 8A3 and 9.8A3, types MF, EF, EFT, EP, EPT: 2 cylinder, 209 cm³ (13 cu in), bore x stroke 55 x 44 mm, OHC with timing belt, carburetor, CD ignition.

Quick answers

What engine oil does a Tohatsu MFS 8 / 9.8 carbureted use?
4-stroke engine oil API SE, SF, SG, SH, SJ, SL or SM; SAE 10W-30 or 10W-40; NMMA FC-W 10W-30/40.
What gear oil does a Tohatsu MFS 8 / 9.8 carbureted use, and how much?
Hypoid gear oil, API GL-4 or GL-5, SAE 80W-90 or #90. Quantity: 320 cm³ (10.8 fl oz). Fill through the lower plug hole until oil spills from the upper hole without bubbles.
What spark plug does a Tohatsu MFS 8 / 9.8 carbureted use?
NGK DCPR6E. Gap: 0.8 to 0.9 mm (0.031 to 0.035 in); limit 1.2 mm (0.047 in).
How often should a Tohatsu MFS 8 / 9.8 carbureted be serviced?
The factory schedule lists: B models, initial 20 hours or 1 month; B models, every 50 hours or 3 months; B models, every 100 hours or 6 months; B models, every 200 hours or 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 Tohatsu MFS 8 / 9.8 carbureted?
The factory schedule says to inspect it at 50 hours, 100 hours and 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 Tohatsu MFS 8 / 9.8 carbureted?
Neutral 950 r/min; trolling in forward 900 r/min (warm engine).
What is the propeller nut torque on a Tohatsu MFS 8 / 9.8 carbureted?
12 N·m (9 lb·ft) [1.2 kg·m], then turn on to line up a groove and fit the split pin.

Maintenance specs

Engine oil4-stroke engine oil API SE, SF, SG, SH, SJ, SL or SM; SAE 10W-30 or 10W-40; NMMA FC-W 10W-30/40
Engine oil drain plug (M14 x 1.5, 16 mm)23.5 N·m (17 lb·ft) [2.4 kg·m]; oil the washer
Gear oilHypoid gear oil, API GL-4 or GL-5, SAE 80W-90 or #90
Gear oil quantity320 cm³ (10.8 fl oz). Fill through the lower plug hole until oil spills from the upper hole without bubbles
Gear case leak test0.069 MPa (10 psi) [0.7 kgf/cm²], must hold for 10 seconds; do not go over this pressure
Spark plugNGK DCPR6E
Spark plug gap0.8 to 0.9 mm (0.031 to 0.035 in); limit 1.2 mm (0.047 in)
Spark plug torque (M12 x 1.25)18 N·m (13 lb·ft) [1.8 kg·m]; hand tight first
Idle speedNeutral 950 r/min; trolling in forward 900 r/min (warm engine)
ThermostatStarts to open 60 ± 1.5 °C (140 ± 3 °F); full open 75 ± 1.5 °C (167 ± 3 °F); lift 3.0 mm (0.12 in) or more
AnodesGear case anode, and clamp bracket anode on PT models: replace when reduced to 2/3 of a new part; retighten the bolts at every inspection; do not oil or paint.
Propeller nut (M10 x 1.25)12 N·m (9 lb·ft) [1.2 kg·m], then turn on to line up a groove and fit the split pin
Valve clearance, coldIN 0.13 to 0.17 mm (0.0051 to 0.0067 in); EX 0.18 to 0.22 mm (0.0071 to 0.0087 in); tappet adjusting lock nut M6: 7 N·m (5 lb·ft) [0.7 kg·m]
Timing beltInspect for cracks, damage and wear; replace if it deflects more than 10 mm when pressed at the center. Timing (drive) pulley nut M22: 50 N·m (36 lb·ft) [5.0 kg·m]; camshaft pulley bolt 11 N·m (8 lb·ft)

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.

  1. B models, initial 20 hours or 1 month

    Replace: engine oil, oil drain plug gasket, gear case oil, gear case plug gaskets, propeller cotter pin

    Check: spark plugs, fuel filter

  2. B models, every 50 hours or 3 months

    Replace: gear case oil, gear case plug gaskets, propeller cotter pin

    Check: fuel filter, water pump impeller, anodes

  3. B models, every 100 hours or 6 months

    Replace: engine oil, oil drain plug gasket, gear case oil, gear case plug gaskets, propeller cotter pin

    Check: spark plugs, fuel filter, water pump impeller, thermostat, anodes

  4. B models, every 200 hours or 1 year

    Replace: engine oil, oil drain plug gasket, 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: Tohatsu 4 stroke 8/9.8 service manual, MFS 8B / 9.8B, OB No. 003-21056-2; Tohatsu 4 stroke 8/9.8 service manual, MFS 8A3 / 9.8A3, OB No. 003-21056-1. Specs can change by model year and serial number, so always confirm your exact model from the serial tag on the clamp bracket.

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