Review: Yamaha F6C three months on

By: Andrew Norton

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Engine Man had plenty of time to review the Yamaha F6C since he reported on his first impressions three months ago.

Review: Yamaha F6C three months on
The Yamaha F6C outboard motor is by far the most powerful four-stroke 6hp engine in Australia when mounted on the correct hull.

Originally published in TrailerBoat #260, August / September 2010.

When I first reported on the Yamaha F6C outboard I mentioned how it outperformed its direct four-stroke competition, including the Suzuki DF6. At the time of compiling the initial report the review engine had clocked up only seven hours and still needed more running-in. Since then the engine has clocked up 19.2 hours and all trials have been completed. On my flat-bottomed Sea Jay 3.4 Punt, spinning the standard 8.25in pitch semi-weedless alloy prop and pushing 290kg, the 139cc F6C averaged 29.7kmh (16.0kts) and 5570rpm at WOT, using 2.3lt/h.

Pushing the same load but spinning a 7in high-thrust prop to suit its taller 1.92:1 gear ratio, my long term evaluation 138cc DF6 averaged 28.0kmh (15.1kts) and 5690rpm using 2.6lt/h.

 

YAMAHA F6C WEIGHT SENSITIVITY

Like all single cylinder four-stroke outboards, the Yamaha F6C is very sensitive to boat weight, hull design and WOT rev rate. On my 3.6m Sea Al Super Skua aluminium dinghy, spinning the same prop and pushing 285kg including two adults and fishing tackle, the F6C trolled us at 5.8kmh (3.1kts) and 1400rpm, using 0.24lt/h. But because the engine didn’t reach the middle of the recommended WOT rev rate range, all it could manage at WOT was a semi-planing 20.5kmh (11.1kts) at 4760rpm, using 2.0lt/h.

On this hull and pushing the same load, my DF6 trolled us at 4.6kmh (2.5kts) and 1260rpm, using 0.20lt/h. However, because the rev rate was above where maximum power is developed, it averaged 23.5kmh (12.6kts) and 5350rpm at WOT, though it used 2.6lt/h to achieve this. Around the 7.5kmh "loop" the Suzy used 0.68lt/h compared to only 0.49lt/h for the overpropped Yamaha outboard.

However, neither the Suzuki or Yammie could match the 165cc two-stroke 6C on this hull. Pushing the same load and spinning a 7.5in prop it averaged 24.7kmh (13.3kts) and 5000rpm but used 3.6lt/h.

With its twin-cylinder crossflow powerhead the 6C had lower vibration levels than the F6C when trolling, and it emitted oil smoke only after half an hour or more at the trolling rev rate. But after two-plus hours of trolling during one trial the F6C hadn’t missed a beat, and during the evaluation period it didn’t blow a whiff of oil smoke.

 

PERFORMANCE AND OIL CONSUMPTION

Although its vibration levels across the entire rpm range were higher than the 6C, at or near WOT the Yamaha F6C was only slightly rougher than the 6C, and it’s the only single-cylinder four-stroke outboard I’ve tested that runs smoother at WOT than at midrange rpm. I suspect that this is more a case of powerhead mounting design than engine balance because, unlike the DF6, the vibration is directed away from the boat and into the tiller arm.

As the engine "freed-up" from the initial evaluation period to completion of testing, the vibration became much more apparent. Between 2000 and 4000rpm the Yamaha F6C transmitted vibration that were sufficient to induce numbness in my wrist and arm after about 15 minutes of operation, whereas in this rev range the DF6 transmitted excessive vibration through the hull but not the tiller arm.

Because the carbie is mounted alongside the cylinder head and not atop it as with the DF6, after a couple of hours at anchor while fishing the F6C didn’t suffer vapour-lock like the DF6, and it started instantly with no throttle opening needed. No doubt this was helped by the air intake inlets in the upper cowl, which allow hot air from the powerhead to escape, unlike the DF6 which has a sealed upper cowl.

Unfortuntaely the integral tank is too opaque to clearly see fuel levels in strong sunlight and the plastic should be more translucent like the DF6 tank.

Because the cooling water intakes are centrally located in the lower unit, small diameter rabbit or flushing ears can be used to freshwater-flush the powerhead. This saves time at the end of a day’s fishing and also reduces water usage compared to using a flushing drum.

At the end of 18 hours of running, which included a total of 7.5 per cent WOT operation, averaging 0.65lt/h the fuel/oil ratio was 1670:1, very good for a single-cylinder four-stroke engine. Over the same period, and with the same WOT percentage, averaging 0.69lt/h the DF6 had a fuel/oil ratio of 885:1, which improved to 2760:1 by 50 hours. Based on the F6C’s initial ratio, by 50 hours the fuel/oil ratio could be as lean as 5000:1!

After a total of 24 hours of saltwater leg/lower unit immersion, no corrosion was apparent anywhere on the review Yamaha F6C.

 

THE VERDICT

Apart from the annoying midrange rpm vibration, which could be reduced by extending the tiller arm and changing the operating angle for a more natural feel, Yamaha has done a great job of improving on most of the features of the DF6 outboard, which for seven years was the best engine in its power range. From my reviewing of all the four-stroke sixes on the Australian market, the Yamaha F6C is by far the most powerful of them when mounted on the right hull, and showcases what can be achieved by Yamaha when it sets out to create the best four-stroke six available.

 

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Originally published in TrailerBoat #260, August / September 2010. Why not subscribe to Trade-a-Boat today?

 


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