Tohatsu MFS 3.5A

Big little outboard

Released in August 2006, the Tohatsu 3.5A provides significantly better performance than Suzuki's 68cc DF2.5. It has a forward-neutral clutch, 360-degree steering, a long tiller arm that folds down alongside the leg, and a thermostatically-controlled, water-cooled powerhead for a long engine lifespan. The 3.5A complies with California Air Resources Board (CARB) 2008 and Australian OEDA three-star emission requirements.

Unlike the Suzuki, it has splash lubrication, which limits the choice of sump oils and the tilt-lock knob prevents the 3.5A's lower unit from clearing the water when the engine is fully tilted. But to counter this, there’s a pilot water discharge flow outlet that's clearly visible at all revs and a push-pull throttle cable for better reliability than the Suzy's single-cable system.

Like the Suzy, the 3.5A has an oil level sight glass in the lower cowl that makes checking the oil level easier than a dipstick, and the external sump oil drain plug is a nice touch. The upper cowl has a baffled air intake to prevent water from reaching the powerhead and, with its rubber sealing ring it is a much stronger moulding than the one on the Suzuki. It can also be completely separated from the lower cowl.

In October I borrowed a 3.5A from Lakeside Marine, the national Tohatsu distributor, for four weeks. Lakeside performed the initial three-hour break-in period, allowing me to operate the engine at wide open throttle for sufficient periods to conduct the necessary performance trials.

Because the fuel tank is mounted on top of the powerhead and gravity-feeds directly to the carburettor, the engine always started first pull, hot or cold. Using Valvoline SAE 10W40 oil, no smoke appeared at any time.

To enable the 3.5A to rev to where it should, the standard seven-inch pitch ‘I7’ prop was soon swapped for the 5.7-inch 'F6' prop. Because Tohatsu's 74.6cc two-stroke M3.5B, which runs the same lower unit and gear ratio, revs to between 4200 and 5300rpm, the I7 prop is perfect for this engine on small car toppers.

Tested on my three-metre deHavilland John 10 punt (similar to Hunter Marine's Fisherman 10) and pushing 225kg, including two adults and fishing tackle, the loan engine averaged 4.4km/h and 0.12lt/h at 1500rpm when trolling, and 12km/h and 1.10lt/h at 5720rpm at WOT. Across a 'loop' averaging 7.5km/h with 10 per cent WOT and 40 percent trolling operation, the 3.5A used 0.57lt/h.

In comparison, on the same hull, pushing the same weight and spinning the I7, a loaned M3.5B averaged 3.3km/h and 0.22lt/h at 1200rpm when trolling, and 12.2km/h and 1.64lt/h at 5180rpm and WOT. Across the 7.5km/h loop, the M3.5B used 0.73lt/h.

Of course, with its four-stroke powerhead and timing advance, the 3.5A ran a lot cleaner and smoother than the 50:1 premix M3.5B, which has fixed timing, while the tiller arm with twist grip throttle control was much easier to use than the M3.5B's short tiller and slide-control throttle. But the 42 per cent greater weight and lack of a carry handle where needed made the 3.5A much harder to carry any distance than the M3.5B.

Averaging 0.39lt/h with a total of 7.5 per cent of WOT operation for the 4.8-hour evaluation period, the fuel/oil ratio was a reasonable 380:1, which should improve to around 3000:1 after about 50 hours.

After a total of 15 hours of saltwater leg/lower unit immersion, the only corrosion apparent was slight surface pitting of the zinc anode.

Powerhead access is reasonable with the oil filler cap, spark plug, carbie and fuel filter easily reached, but checking valve clearances would be difficult as the rocker cover is well down inside the lower cowl. Recommended servicing intervals are every 50 hours or six months after the initial 20-hour check-up. The waterpump impeller should be replaced every 100 hours or yearly.

The MFS 3.5A has a three-year warranty for recreational usage.