Yamaha 70B outboard engine review

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

Originally 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 also 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.

YAMAHA 70B

Available in only one model with longshaft and remote control, the OEDA one-star-rated Yamaha 70B develops 69hp at 5500rpm from its three-cylinder loopcharged 849cc powerhead and has a WOT range of 5000-6000rpm. The engine is fitted with electronic ignition-timing advance and a cold-start fuel primer, eliminating the need to fiddle with electric chokes, but 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 outbords, including the classic Johnson 70 (discontinued in 2000). Powerhead access on the Yamaha 70B is excellent, with the bowl-type fuel and oil filters 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 the time 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 did maintain a clean plane at relatively low revs. The prop refused to let go through tight turns at 4000rpm and the acceleration was better than 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.

THE TRADE-A-BOAT 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.

As of April 2013 the Yamaha 70B has a price of $9346, with a spare stainless steel prop around $800. 

Thanks to Yamaha Motor Australia (69 River Gate Place Murarrie, Qld 4172, 07 3906 7041) for the loan of the 70B for this test.

YAMAHA 70B ENGINE PERFORMANCE

1.9kts (3.5kmh) @ 700rpm (trolling) — 1.9L/h
14kts (26kmh) @ 2800rpm (min. plane) — 8.7L/h
23.1kts (42.9kmh) @ 4000rpm (cruise) — 11.4L/h
36.9kts (68.4kmh) @ 5700rpm (WOT) — 27.5L/h

THE COMPETITION

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