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Tohatsu M8B / M9.8 two-stroke (169 cc twin) service guide

Oil, gear oil, spark plugs and the service schedule for the Tohatsu M8B / M9.8 two-stroke (169 cc twin) (8 to 9.8 hp, two-stroke), from the factory service manual, plus how we service it in Hawaiʻi.

Models covered: Tohatsu M8B. 2 cylinders, 169 cc (10.3 cu in), bore 50 x stroke 43 mm, 8 PS at 5000 rpm, full throttle range 4,500 to 5,500 rpm, mixed gasoline (gasoline 50 : oil 1), forced water cooling with a rubber impeller and a thermostat (pressure relief valve), recoil start (electric starter optional), flywheel magneto C.D. ignition, Keihin carburetor, dog clutch forward, neutral and reverse, reduction 13:27, 13 L separate tank. Premix, no oil injection.

Quick answers

What two-stroke oil does a Tohatsu M8B / M9.8 two-stroke use?
Tohatsu oil.
What is the fuel to oil ratio for a Tohatsu M8B / M9.8 two-stroke?
Gasoline 50 : oil 1. Break-in after engine parts are replaced: 20:1; M 2-takt gives 25:1 for break-in.
What gear oil does a Tohatsu M8B / M9.8 two-stroke use, and how much?
Genuine Tohatsu gear oil #80. Quantity: About 320 cc.
What spark plug does a Tohatsu M8B / M9.8 two-stroke use?
NGK BP7HS-10 or Champion L82YC, qty 2. Gap: 0.9 to 1.0 mm (0.035 to 0.039 in); limit 1.2 mm.
How often should a Tohatsu M8B / M9.8 two-stroke be serviced?
The factory schedule lists: Initial 10 hours; Every 50 hours; Every 100 hours; Every 200 hours or annually. 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 M8B / M9.8 two-stroke?
The factory schedule says to replace it at 200 hours, and we follow that. In any case we never let it go longer than a year in Hawaiʻi's salt water.
What is the idle speed of a Tohatsu M8B / M9.8 two-stroke?
750 ± 50 rpm with the clutch ON (forward). M 2-takt: 750 rpm in gear, 950 rpm in neutral.
What is the propeller nut torque on a Tohatsu M8B / M9.8 two-stroke?
Standard pitch S: 8.5, L: 7.5, UL: 7.5.

Maintenance specs

Two-stroke oilTohatsu oil
Fuel to oil mixGasoline 50 : oil 1. Break-in after engine parts are replaced: 20:1; M 2-takt gives 25:1 for break-in
Gear oilGenuine Tohatsu gear oil #80
Gear oil quantityAbout 320 cc
Gear oil upper and lower oil plugs2 plugs.
Spark plugNGK BP7HS-10 or Champion L82YC, qty 2
Spark plug gap0.9 to 1.0 mm (0.035 to 0.039 in); limit 1.2 mm
Spark plug torque2.5 to 3.0 kg-m (18.1 to 21.7 lb-ft), 14 mm
Idle speed750 ± 50 rpm with the clutch ON (forward). M 2-takt: 750 rpm in gear, 950 rpm in neutral
ThermostatWax type; starts to open at 50.5 to 53.5 °C, fully open at 65 °C
AnodesCheck for wear and corrosion every use; replace a wasted anode
PropellerStandard pitch S: 8.5, L: 7.5, UL: 7.5.

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. Initial 10 hours

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

  2. Every 50 hours

    Replace: gear case plug gaskets, propeller cotter pin

    Check: spark plugs, fuel filter, gear case oil

  3. Every 100 hours

    Replace: gear case plug gaskets, propeller cotter pin

    Check: spark plugs, fuel filter, gear case oil

  4. Every 200 hours or annually

    Replace: gear case oil, gear case plug gaskets, propeller cotter pin, water pump impeller

    Check: spark plugs, fuel filter

Source: Tohatsu M8B service manual, JULY 1987. 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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