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Yamaha 15hp or 20hp Motor Question.

Old 10-17-2016, 08:15 AM
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Default Yamaha 15hp or 20hp Motor Question.

Can someone answer me this?

Looking at the Yamaha 15hp and their spec sheet, the 15/20hp, the bore/stroke are the same as well as the gear ratio.

Why then can't one buy a 15 and bump it up to a 20?

Is the computer controlled module making or setting the HP of these two motors, or is it the carb jetting?

Can one buy a 20 module and thus have a 20HP motor?

Am I missing something cause everything seems the same as far as specs go.

Specs for both 15/20: http://yamahaoutboards.com/outboards...specifications

Displacement 22.1 ci (362 cc)
Bore x Stroke 63 x 58.1mm (2.48 x 2.29 in)
Prop Shaft Horsepower 15hp @ 5500rpm
Compression Ratio 9.3:1
Intake Carburetor
Ignition CDI Micro Computer
Spark Plug DPR6EB-900-000
Alternator Output 10A **
Gear Ratio (27:13) 2.08:1

Here is the link to their module: http://yamahaoutboards.com/outboards/Portable/benefits

Thanks,
Jim
Old 10-22-2016, 03:22 PM
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Any thoughts?

Any mechanics here want to speculate?

Thanks,
Old 10-22-2016, 05:16 PM
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It's in the carb.

Smart money buys a Suzuki with EFI! Those little carbed Yamahas are finicky.
Old 10-22-2016, 05:19 PM
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It's typically in the carb AND the exhaust.

It won't be cost effective imho.

It is true that modern carb passages (on all brands) are smaller than frog hairs.

If you feed them clean fuel no problems . . .

Suzuki is probably cheaper and is a great product
Old 10-23-2016, 06:05 AM
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Thank you. I was thinking about this and I also conclude it is the carb/jets etc.
I'll look into this again.
Thanks.
Old 10-23-2016, 06:25 AM
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It is not JUST in the carbs/jets!

I tried to do this with an F6 to an F8 it is not feasible.

Look at the parts lists side by side to see the difference.

The F15 and F20 have different intakes, cylinder heads and exhaust among other things.
Old 10-23-2016, 11:01 AM
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Ok thanks and I see what you mean. I'll bet you can got up one jet size and gain a little but I'm thinking of a regular car engine etc. That is where I was coming from/thoughts. Opening up the breather etc but I guess once you get into it, might as well just buy a bigger motor.

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