Review: Tohatsu M2.5A Marine Engine

Review of the lean burning Tohatsu M25A Outboard

Can a carbie two-stroke be made to run lean?

The answer is yes, when an engine is significantly de-rated from its maximum designed output.

Introduced to the Aussie market in 1988, the Japanese-made direct drive M2.5A was imported by Lakeside Marine, the national Tohatsu distributor until 2015. Its WT47B power head was also used in the Mercury 2.2 released in 1985, the Mariner/Mercury M2.5 in 1990 and the Force 3 in 1991. All of these engines were  direct drive and were designed for transom heights to 16 inches.

THE NITTY GRITTY

The WT47B (water cooled two-stroke 47-millimetre cylinder-bore B series) power head had just one piston ring and a single jet carbie with four needle jet positions to make the engine run richer or leaner. The throttle and choke were controlled by simple levers connected directly to the carbie. A 1.4-litre integral tank gravity fed the carbie via a fuel cock. 

Originally the engine had a Mitsubishi Electric flywheel magneto with breaker points, but in 1991 this swapped to CD ignition. The ignition timing was fixed at 20 degrees Before Top-Dead-Centre, whereas timing advance would normally range from five degrees BTDC to 25.

The M2.5A had a massive Wide Open Throttle range of 1,400rpm and though maximum torque was down from the M3.5A the M2.5A still produced 4.1 newton metres at 3,700rpm and 3.9 at only 3,000rpm. Combined with the availability of four plastic and two alloy props, with pitches varying from 4.3 to 7.0 inches, the M2.5A suited a wide variety of roles.  

Four trim positions and a full tilt stopper knob were provided and the lower unit incorporated a water pump impeller in the gear case, just ahead of the prop. Remove this and the end cap and the impeller could be easily replaced without dropping the lower unit, an idea adopted by Suzuki in its DT2 and DT2.2.

Early models required a ten-hour break-in on 20:1 before switching to the normal 50:1 but even running such a rich mix the first M2.5A I borrowed in 1989 emitted very little oil smoke. More recent models required only a five hour break-in on a 25:1 mix but unusually no WOT operation during that period.

Originally the first service was at 10 hours then every 50 hours or six months, but more recently this was changed to 20 hours or three months then 100 hours or annually with water pump impeller replacement. However, as the impeller rotated at only prop shaft and not drive shaft rpm it had a very long service life.

Right through to its discontinuation in Oz the M2.5A retained the clamshell cowl from the original M3.5A. In later years this affected the engine's popularity because buyers wanted lift-off cowls. If one half was removed at a time power head access was tolerable but remove both halves and re-assembly required a lot of patience. At least the spark plug was accessed via its own clip-on hatch and the well placed carry handle made lugging around the M2.5A very easy.

ON THE WATER

I hadn't tested an M2.5A since 1996 so for the latest test Lakeside Marine found me the last new engine in Oz. A 2008 model, it had been stored unused in its shipping case for ten years. Lakeside's Trevor Robinson checked the gear oil level, fitted a new spark plug, put the engine in the test tank, added a 25:1 mix, cranked the engine and it fired up immediately. Not bad after a 10 year slumber!

Carefully run in for five hours, the engine started “freeing up” after about two hours with lower vibration levels, which were initially high enough to shake the steering friction adjuster knob overboard. Fortunately an occy strap attached to the carry handle prevented the engine from swivelling 180 degrees when tilted. The quick release trim rod made changing trim positions very easy.

On my 2003 flat bottomed Sea Jay 3.4 Punt the loan M2.5A didn't have as much top end grunt as a Mercury four-stroke F2.5 tested under identical conditions. While from one third throttle opening out to WOT the M2.5A was quieter than the F2.5, vibration levels were higher with the hull visibly vibrating at certain rpm. But on the 15-inch transom, set on the second trim hole the M2.5A could be swivelled 180 degrees for reverse without fear of the leg hitting the transom, so controlling the Sea Jay alongside a jetty was a cinch.

On my 1981 3.8-metre Fairlite Gull sailing dinghy the M2.5A returned even better performance and fuel efficiency. However, below two-thirds throttle opening the foredeck shook visibly, so pottering around bays was out. Set on the second trim position for best all-round performance the leg contacted the 16-inch transom when swivelled 180 degrees so there was no provision for reverse.

The lean jetting required a fair amount of choke to be used for cold starts where about as much effort as the Merc was needed due to the engine being always in gear. Running on semi-synthetic Valvoline oil and premium (95) unleaded from new oil smoke emissions were low, even on 25:1.  At all speeds the pilot water discharge or cooling water tell tale was clearly visible.

THE WRAP

The last batch of Oz M2.5A models was dispatched to dealers in September of 2015, with a RRP of $790, but according to Tomos Marine of Marks Point NSW, a 2008 model in good condition is worth around $650, while a 1988 model still fetches $100. So providing the engine has been maintained it holds its value. And it has excellent salt water corrosion resistance. After a total of 130 hours of salt water leg/lower unit immersion the 1996 loan engine had only slight surface rust on the stainless steel prop shaft and fuel line clamps. 

The combination of simple but solid engineering and a great choice of prop pitches ensures this outboard will always find a place on small inflatables and tinnies and displacement dinghies. It's ideal for cruising yachties who need the ultimate in reliability and straightforward servicing in a fuel efficient and lightweight package. 

SEA TRIALS

Single M2.5A and F2.5 on Sea Jay 3.4 Punt, pushing a total of 275 and 280kg respectively, including two adults and fishing tackle. Average of two way runs on Lake Macquarie NSW, calm water. Range is in nautical miles with a 10% reserve, rounded up or down to nearest whole number, as are the decibel readings.

THROTTLE OPENING: TROLLING

THROTTLE OPENING: ONE THIRD

*Note higher dB readings than F2.5 when trolling due to engine vibrating more.
Data supplied by author

SPECIFICATIONS
Tohatsu M2.5A
ENGINE TYPE Loop charged single cylinder two-stroke outboard
RATED BHP/MHP* 2.4/2.4 at 4500rpm
WOT rpm range 3800 to 5200rpm
DISPLACEMENT 74.6cc
BORE x STROKE 47 × 43mm
GEAR RATIO 1.85:1
DRY LONG SHAFT WEIGHT 12.5kg
RRP NEW (2016) $790
OEDA stars 0

* Brake horsepower/metric horsepower or PS