REVIEW: TOHATSU M3.5B VS MFS3.5B

By: Andrew Norton

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Simplicity to small motor sophistication...

REVIEW: TOHATSU M3.5B VS MFS3.5B
TOHATSU M3.5B and MFS3.5B

The original M3.5B was released on the Oz market in 1991 and since then little has changed. Based on the direct-drive M3.5A introduced in 1975 it has a clutch and a deeper 2.15:1 gear reduction. The 50:1 engine has only one piston ring and ignition timing fixed at 20 degrees Before Top Dead Centre. A 1.4lt integral tank gravity feeds to single-jet carbie and the throttle is a simple bar lever next to the choke lever.

Maintaining the M3.5B is a pain because the clamshell cowling needs to be carefully screwed together. A separate hatch accesses the spark plug. Servicing intervals are every 50 hours or annually after the first service at 20 hours. The break-in period is five hours on 25:1.

Because of the fixed ignition timing the engine runs too advanced at low rpm and has fairly high vibration levels. It’s best run as much as possible at about two-thirds throttle opening where the fixed timing gives maximum fuel efficiency.

The MFS 3.5B was released in 2006 as the MFS 3.5A and was Tohatsu’s first four-stroke under 5bhp. It also has a clutch and the same reduction ratio as the M3.5B but develops its maximum output much higher up the rpm range. However it has a twin-jet carbie and electronic timing advance from three to 25 degrees BTDC so it’s a much sweeter-running engine. The single-cylinder OHV engine has gear-driven push rods and splash lubrication as there’s no shallow-water drive setting to reduce lubricating efficiency. A thermostatically controlled cooling system helps the MFS 3.5B gain an OEDA 3-star emissions rating whereas the M3.5B has a "0" rating.

Because the 1lt fuel tank is mounted atop the engine, access to the rocker cover for checking valve clearances is limited. At least the cowl lifts off the engine pan for carbie and spark plug access and a neat little elbow under the engine makes draining the 0.3lt sump easy. A twist-grip throttle and pull-out choke knob make operating the MFS 3.5B easier, but like the M3.5B there are four trim positions and a stopper knob for full leg tilt. Both engines are designed for 16-inch transom heights.

The recreational usage warranty is three years for both engines.

SEA TRIALS

Single M3.5B and MFS 3.5B on 1978 flat-bottomed 3m deHavilland John 10 punt, pushing a total of 220kg and 225kg and swinging 7-inch and 5.7-inch plastic weedless props respectively. Average of two-way runs on Lake Macquarie, NSW, in calm water. Range is in nautical miles with a 10% reserve.

ENGINE M3.5B MFS 3.5B
TROLL RPM 1200 1500
KTS 1.8 2.4
LT/H 0.2 0.1
RANGE 11 21
2/3 THROTTLE 4100rpm 4500rpm
KTS 5.9 5.8
LT/H 1.1 0.8
RANGE 6 7
WOT RPM 5200 5700
KTS 6.6 6.5
LT/H 1.6 1.1
RANGE 5 6

"Loop" of cruising with 10% WOT and 40% trolling operation, averaging 4kts

LT/H 0.73 0.57
RANGE 6 6

SPECIFICATIONS: TOHATSU M3.5B

ENGINE

TYPE Loop-charged two-stroke
Rated BHP/MHP* 3.5/3.5 at 4750rpm
WOT range 4200 to 5300rpm
Displacement 74.6cc
Bore x stroke 47mm x 43mm
Gear ratio 2.15:1
Dry weight 13kg
RRP $944
OEDA stars 0
*Brake horsepower/metric horsepower

SPECIFICATIONS: TOHATSU M3.5B

ENGINE

Type OHV Four-stroke
Rated BHP/MHP* 3.5/3.5 at 5500rpm
WOT range 5000 to 6000rpm
Displacement 85.5cc
Bore x stroke 55mm x 36mm
Gear ratio 2.15:1
Dry weight 18.4kg
RRP $1309
OEDA stars 3
*Brake horsepower/metric horsepower

Check out the full review in issue #502 of Trade-a-Boat magazine. Subscribe today for all the latest boat news, reviews and travel inspiration.

 


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