Review: Tohatsu MFS9.8A3 Four-Stroke Outboard Motor

The main issue with portable four-stroke outboard motors around 10hp is their weight compared to the two-stroke boat engine competition. One four-stroke 9.9hp outboard weighs in at 40kg and on a 3.7m tinnie that can create serious freeboard issues aft, particularly on a short shaft transom.

On hulls of this size, every kilogram lighter can make a difference and at 37kg the Tohatsu 9.8 is lighter than one competitor's four-stroke 8hp ‘portable’ marine engine. Mind you, it's still 42 per cent heavier than the two-stroke Tohatsu M9.8B outboard motor, so freeboard at the transom can be adversely affected.

There's no possibility of just a helmsperson (unless they're vertically challenged) using this engine without compensation from a fishing mate or water ballast forward to even out fore and aft trim.

However, once the increased weight aft is compensated for, opting for the four-stroke motor can dramatically improve fishing pleasure. The four-stroke Tohatsu is more fuel efficient than the M9.8B, vibrates less when trolling (great for porkers who want to retain their bulk) and is quieter, particularly at wide open throttle (WOT). It's just not as quick at WOT.

Servicing is more expensive, but if you're an angler who uses the boat frequently, over time the higher outlays can be offset by lower fuel and oil costs. 

The Tohatsu MFS9.8A3 is typical of four-stroke outboard motors in its power range: twin cylinders with a belt driven overhead camshaft operating four valves. It has effective thermostatically controlled water cooling and desirable engine protection features such as a rev limiter that cuts in at 6250rpm and automatic reduction to 3000rpm should the oil pressure fall off.

When the engine was first released on the Australian market in 2004, it had a manual choke that was cross linked with the throttle for more positive starting, but this has been replaced with an automatic choke system in line with Tohatsu's four-stroke 15 and 20. Also the side gear shift has been replaced with an upfront shift positioned so the long tiller arm can fold back against the upper cowl. Great for transporting the engine in a small car boot.

The Tohatsu four-stroke shares many features with its two-stroke counterpart, including six trim positions, one shallow water drive setting, a strong full tilt lock and an automatic reverse lock. The lower unit is also straight from the Tohatsu M9.8B engine, allowing the same range of props to be fitted.

A 12V, 6A alternator is available but as it lacks voltage regulation a 70A/h battery should be used to prevent this “frying” on long runs to and from a favourite fishing spot.

Maintenance and access

Power head access is very good with the spark plugs, fuel filter and carbonator easily reached. The oil sump holds 0.8L, a bit small for a 9.8 so I recommend changing the oil every 50 hours or six months, especially as the oil can be diluted during extended trolling operation. Lakeside Marine, the national Tohatsu distributor, recommends servicing the engine every 100 hours or six months. The loan engine used Valvoline SAE 15W40 oil and returned low oil consumption on this product.

The recreational usage warranty is three years.

Performance on the water

The well run-in engine I borrowed was also supplied with a tachometer wired-in and mounted in a neat plywood box. How handy to have a tacho ahead of me that could be easily read underway, instead of using a portable high tension lead unit. The engine started easily hot or cold with a firm one-hand pull and the only time oil smoke appeared was after the engine had been transported on its side.

Mounted on my 1988 3.6m Sea Al Super Skua tinny, the loan engine performed well but did suffer the curse of many smaller four-strokes where the engine is mounted further aft from their transom brackets than comparable two-strokes. This located the leg and lower unit further out resulting in excessive drag. Combined with the additional weight on the transom of the Sea Al the standard 9.5 inch pitch prop fitted to the M9.8B was simply too big and the engine “locked up” at 5000rpm at WOT. Swapping for the same 8.3 inch prop I use with my Tohatsu M8B made all the difference.

The four-stroke outboard trolled effortlessly for extended periods with scarcely any flab reduction qualities and accelerated easily onto the plane, if not as quickly as the M9.8B.Up on the plane, vibration levels remained low and the engine was so quiet compared to its two-stroke mate. However pushing a similar load the M9.8B cruised at 14.4kt and 4700rpm using 3.6L/h, with WOT averages of 19.2kt and 5700rpm gulping 5.3L/h, so the four-stroke is the way to go when top end performance is not essential.

After a total of six hours' testing with a total of 7.5 per cent WOT operation, the MFS9.8A3 averaged an economical 1.15L/h with an excellent fuel/oil ratio of 1150:1.

The Trade-a-Boat verdict

Tohatsu outboards has done an excellent job of developing a small twin cylinder four-stroke outboard. It's perfectly suited to anglers who care about the environment and/or mainly fish fresh water lakes and rivers, where I personally believe carbie two-strokes should be banned. But I would mount it on a flat-bottomed or vee nose punt which has more buoyancy aft.

Contact Lakeside Marine on (02) 4392 6110

Tohatsu MFS9.8A3 sea trials and performance

Single Tohatsu MFS9.8A3 outboard motor on a 3.6m Sea Al Super Skua tinny, swinging 8.3 inch alloy prop and pushing 285kg including two adults and fishing tackle. Average of two way runs on calm water. The best planing fuel efficiency was at 5000rpm.

900rpm reaches a speed of 2kt with fuel burn of 0.3L/hr and economy of 0.15L/nm

4400rpm reaches a planing speed of 12.7kt with fuel burn of 2.8L/hr and economy of 0.22L/nm

5900rpm (WOT) reaches a maximum speed of 18.4kt with fuel burn of 4.1L/hr and economy of 0.22L/nm

Tohatsu MFS9.8A3 specs

Price      TBA

Type      Twin cylinder four-stroke outboard motor

Rated BHP/MHP*            9.7/9.8 at 5500rpm

Rec. WOT range                5000 to 6000rpm

Displacement    209cc

Bore x stroke     55 x 44cc

Gear ratio            2.08:1

Weight 37kg (dry, short shaft)

OEDA stars          3

* Brake horsepower/metric horsepower.