Tohatsu M90A

Released locally 18 years ago, the M90A is unique in Tohatsu's carburetted two-stroke outboard range, in that it is neither an up-rated or de-rated version of another engine with the same piston displacement. The M90A falls midway between the piston displacement of the 1140cc Yamaha 90A and the 1386cc Mariner/Mercury 90. The Tohatsu M90A is 10 per cent heavier than the Yamaha but two per cent lighter than the Merc. All these engines have three cylinders and three carbies.

As with all of its three- and four-cylinder carbie counterparts, the M90A utilises Tohatsu's "automixing" variable ratio oil injection system, where oil is injected at the fuel pump, ensuring it passes through the carbies and lubricates the throttle butterfly valves. To prevent oil separating from petrol in the carbie bowls during extended periods of disuse, Isopropyl Alcohol (such as Bombardier 2 + 4 Fuel Conditioner and Stabiliser) can always be added to the petrol before storing the engine. Tohatsu's system is inherently more reliable than Yamaha's, where oil bypasses the carbies and is injected at each set of reed valves through small diameter oil delivery lines.

However the M90A is showing its age in that a lower cowl-mounted tilt switch is not fitted and to operate the trim/tilt system the ignition switch must be left on, creating a misleading amount of engine hours should an hour meter be fitted.

Mounted on a Southern Star 485 SF cuddy cab, spinning a 15in-pitch Tohatsu alloy prop and pushing a total of 980kg including two adults, the demo M90A returned reasonably quick cruising and Wide Open Throttle speeds. It started instantly hot or cold and despite running on a 50:1 break-in premix in addition to the oil injection, it emitted oil smoke only below 1500 revs. As expected, the engine was a bit chuggy below 1500 revs and not as smooth-running as its TLDI 90 counterpart, but above 2000 revs there was little difference in vibration or noise levels.

Providing the antiventilation was kept at least three-quarters immersed, power astern was very good and no cooling water starvation occurred. Using my handheld Lowrance GPS and recording the average of two-way runs, the trolling average was 4.3km/h on 800 revs and at 2000 revs we averaged 12.1km/h. A clean plane was achieved at 27.8km/h on 3000 revs and at these revs the hull stayed fully on the plane into a 25km/h headwind and 30cm chop. Cruising quietly at 4000 revs the average speed was 45.1km/h using 15.5lt/h and through full lock figure of eight turns at these revs no prop ventilation occurred.

The WOT average was 62.2km/h on 5500 revs using 35.0lt/h and at these revs we could still talk normally at the helm. Maintaining the M90A is straightforward, with servicing intervals recommended every 50 hours or six months after the first 10 hours. The water pump impeller should be replaced every 100 hours or yearly.

For saltwater anglers who want a relatively inexpensive and easily-maintained engine for a compact cuddy cab, the M90A is very hard to pass up. The warranty coverage is two years for recreational usage.