With its smooth-running powerhead, the Yamaha 70B outboard motor won’t dislodge dentures or loose fillings.

Initially released in 1983, the 70B was the second of Yamaha’s carbie two-stroke outboard motors fitted with variable ratio oil (VRO) injection after the 90A, which is still available. Known as Precision Blend, this system differed from OMC’s VRO in that oil was injected at the reed valves, bypassing the three carbies and allowing them to run straight petrol, thus eliminating the need to run an engine “dry” at the end of a day’s fishing. It also had a powerhead-mounted oil tank, saving valuable onboard space in a small hull.

Precision Blend varied the fuel/oil ratios from 50:1 at WOT to 200:1 when trolling, so with a mix of trolling and cruising, the 2.8L oil tank lasted a helluva long time.

Yamaha 70B

Available in only one model with longshaft and remote control, the OEDA one-star-rated Yamaha 70B develops 69 hp at 5500 rpm from its three-cylinder loopcharged 849 cc powerhead and has a WOT range of 5000- 6000 rpm. The engine is fitted with electronic ignition-timing advance and a cold-start fuel primer, eliminating the need to fiddle with electric chokes. Still, the 80W alternator is a bit weak and needs upgrading in line with the direct competition. At least it has voltage regulation to prevent frying the starter battery heading to and from a fishing spot.

The 2.33:1 gear ratio enables Yamaha’s well-designed K-series props to be fitted, and at 105kg (dry), the engine is lighter than all other available 70hp outboards, including the classic Johnson 70 (discontinued in 2000). Powerhead access on the Yamaha 70B is excellent, and the bowl-type fuel and oil filters are easily cleaned. Servicing is recommended every 100 hours or annually after the first 20, and the recreational-usage warranty is three years.

 On The Water

I have tried several Yamaha 70B outboard motors on a variety of hulls since my first review, but the best combination was the old Haines Hunter 445R runabout (rated to 90hp) with the 70B engine swinging a 19in stainless steel prop. At the time, I also had a Johno 70 on this hull, and it had a slight performance edge, spinning the standard 17in alloy prop to match the 2.42:1 gear ratio, but the premix 50:1 engine was nowhere near as clean-running, and its electric chokes made it finicky to cold start.

Despite running a 50:1 break-in mix in addition to oil injection, the review Yamaha 70B blew oil smoke only on cold starting. The primer gave instant starting, and the engine warmed quickly from cold. The remote-control box had a smooth yet positive shifting action with barely a clunk into forward or reverse. Provided the anti-ventilation plate was kept at least three-quarters immersed power astern was good.

Trolling with hardly any vibration, the Yamaha 70B reminded me of when carbie two-strokes had to troll and perform well, long before midrange four-stroke outboards were available.

Pushing a total of 750kg, the Yamaha 70B was a bit slow coming out of the hole, but it maintained a clean plane at relatively low revs. The prop refused to let go through tight turns at 4000 rpm, and the acceleration was better than that of any comparable-output four-stroke outboard to WOT. The 70B was surprisingly quiet at or near WOT, and no vibration was felt through the little 445R.

Our Verdict

Thank goodness the Yamaha 70B outboard motor is still available. This and the 90A are classics that saltwater anglers should treasure. These engines have had excellent saltwater corrosion resistance ever since Yamaha’s titanium-content YDC30 alloy was introduced in 1995, and are simple to maintain and service.

Thanks to Yamaha Motor Australia, Murarrie, Qld 4172 for the loan of the 70B for this test.

 

Yamaha 70B Engine Performance

1.9kts (3.5kmh) (trolling)

700rpm

1.9L/h

14kts (26kmh) (min. plane)

2800rpm

8.7L/h

23.1kts (42.9kmh) (cruise)

4000rpm

11.4L/h

36.9kts (68.4kmh)

5700rpm (WOT)

27.5L/h

 

The Competition

Outboard

Mercury 75

Tohatsu M70

Weight (longshaft)

138kg

115kg

Cyl/ HP / rpm

3 / 75.1 / 5000

3 / 69 / 5500

Displacement

1386cc

938cc

Warranty (yrs)

5yrs

3yrs

OEDA Stars

1

1

Originally published in TrailerBoat #294, April/May 2013.

 

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This article was written by Andrew Norton and previously appeared at TradeaBoat.com.au, now powered by Only Boats.


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