Tohatsu M8B (October 2005)

For 18 years Tohatsu's M8B has been a consistently good performer, reports Andrew Norton

Released on the Aussie market in 1987, the 169cc M8B replaced the long-running 137cc Tohatsu M8A, which had twin contact breaker points and frankly was a weak engine for its output.

I briefly tested an M8B in 1987 then three years later "lived" with one for a full year. Both engines proved entirely reliable, and although not quite as powerful as the Johnson 8hp engine, were a lot more fuel efficient. The second engine had electric starting in addition to the overhead recoil starter whereas the Johnno is available only with manual starting.

But with Tohatsu Corporation concentrating on development of compact four-stroke outboards in the past few years I wanted to see how the M8B had changed during the 14 years since I had last tested one, especially as this model is also rebadged as the Mariner/Mercury 8 LW. Lakeside Marine, the national Tohatsu distributor, provided me with a fully run-in M8B for evaluation which could be tested immediately.

As expected the powerhead was basically unchanged but other subtle improvements had been made. The upper cowl now has rounded corners and the lower unit a zinc anode trim tab beneath the antiventilation plate to reduce steering effort and improve lower unit protection in saltwater. The cooling water intakes are located directly atop the gearcase torpedo for improved pickup in broken water and have effective removable screens.

Bonding wires were added between the lower unit, leg and steering pivot tube to increase the effectiveness of the larger anode and unpainted stainless steel is used for components such the shallow water drive and full tilt lock support bracket and lever. Fortunately the steering angle is still a useful 130 degrees and the alloy lower cowl and latch of earlier models have been retained.

TOHATSU VS JOHNSON
On my 3.6m Sea Al Super Skua tinnie and pushing a total of 285kg including two adults and fishing tackle, spinning the standard 9in-pitch alloy prop the loan Johnno averaged 32.4kmh at Wide Open Throttle on 5850rpm using 5.1lt/h, whereas spinning the standard 8.3in prop the lone M8B averaged 28.5kmh on 5130rpm using 4.2lt/h.

Cruising at the minimum clean plane speed of 26.8kmh and 4880rpm, the Johnson used 4.2lt/h compared to 24.8kmh on 4600 revs and 3.4lt/h for the Tohatsu. At Dead Slow Troll the Johnno averaged 3.0kmh on 730 revs using 0.6lt/h while the Tohatsu averaged 3.6kmh on 780 revs using 0.5lt/h.

And over a 'loop' of 20 per cent WOT operation averaging 15.0kmh, the Johnno used 3.2lt/h compared to only 2.2lt/h for the Tohatsu under identical test conditions.

DESIGN AND OPERATION
With its loopcharged powerhead, the M8B was nowhere near as smooth running when trolling as the crossflow Johnson, although at WOT the M8B was marginally quieter. Running on standard ULP and semi-synthetic TC-W3 Valvoline High Performance Outboard Oil (recommended by Lakeside for Tohatsu TLDI engines) oil smoke all but disappeared after the first few minutes of operation.

The side-mounted gearshift is not as easy to use as the upfront shift of the Johnson, but because the tiller arm is mounted on the side and not centrally it folds back alongside the upper cowl, making the M8B easier to carry.

Maintaining the M8B is straightforward and Lakeside recommends servicing it every 50 hours or six months after the first service at 10 hours. The waterpump impeller should be replaced every 100 hours or once a year.

For saltwater anglers who drift-fish and don't need an engine capable of trolling for hours on end, the M8B is a very cost-effective alternative to a four-stroke eight. The warranty coverage is two years for recreational applications.

TOHATSU M8B

Engine type: Loopcharged twin-cylinder two-stroke

Prop hp @ rpm: 7.9 @ 5000

WOT rev range: 4500—5500

Piston displacement (cc): 169

Bore x stroke (mm): 50 x 43

Ignition system: CD w/ mechanical timing advance

Charging circuit: Optional 12V 6A

Break-in period (h): 10

Fuel type: ULP 91 RON

Fuel capacity (lt): 12 plastic remote

Oil type: TC-W3

Fuel/oil ratio: Premix 50:1 after 25:1 break-in

Gear ratio: 2.08:1

Transom height (in): 15/20/25

Weights (kg): 26/27/28

Rec. shortshaft retail: $1881

Spare prop: $167

Servicing costs: Year One $329, Year Two etc $224

As per manufacturer's recommended schedule but excluding parts. All prices current as of September 2005. Prop and servicing prices from Coast-to-Coast Boating, Morisset, NSW, tel (02) 4970 5541

Published : Saturday, 1 October 2005
Issue : October 2005

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Prices and specifications supplied are for the market in Australia only and were correct at time of first publication. BoatPoint Australia makes no warranty as to the accuracy of specifications or prices. Please check with manufacturer or local dealer for current pricing and specifications.