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Old 20 April 2020, 10:09   #1
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9.9 to 15HP

Hello all

I'm looking at a project that somebody has asked me to help with.

Ideally it will need the holy grail cheap short shaft 2-stroke outboard.
Annoyingly my lads sold one recently to fund a 25HP

To give us more chances of sourcing what we need I'm happy to convert a 9.9 to 15HP which I've done in the past.

I can dig out my old notes to reacquaint myself with which Suzuki DT9.9s can be converted, and which parts to change, but as there is no brand loyalty involved we'll look at any brand.

So what brands does anyone know this can be done with?

I'm 90% certain it can be done with Mercury 9.9s by changing the carb jetting or complete carb, but think you don't get the full 15HP without changing the exhaust tube.

Can anyone confirm please.

And what about Yamahas etc.

We're also happy to carry out a bit of a re-build.

Thanks

Nasher
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Old 20 April 2020, 10:45   #2
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Hi Nasher, if you go Mariner/Mercury you will need a 10hp not a 9.9hp. Externally they look the same but they are different CC.
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Old 20 April 2020, 11:00   #3
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Country: UK - England
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Mercury/Mariner 2-stroke 10/15HP around late 1990s to 2006 models. I know for sure on these you need a complete new carb as the 15hp has a larger bore carb... I've owned both HPs at same time to compare. Can't remember if any timing changes needed but a manual would tell you. There were some variations of exhaust tube but this may have been more related to model year rather than HP... not sure.

The Yamaha 9.9/15HP of a similar age range (like the one I have) are the very easiest with just a new larger opening reed set needed... or with the measurements (I have them in the manual) you can just open out the reed stops. I assume the reeds themselves are the same so they can flex to the larger opening. The throttle stop might just need adjusting to get the carb wide open... easily checked/adjusted.

Tohatsu you probably won't want as the 2-stroke 9.9/15HP versions are a bit heavy... and remember the 9.8 can't be made into a 15HP.
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Old 20 April 2020, 11:05   #4
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Thread here re my Mercury carbs with photos...

https://www.rib.net/forum/f36/mercur...5hp-68735.html
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Old 20 April 2020, 11:22   #5
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Reed opening measurements for Yamaha are... 9.9HP 1.3mm and 15HP 6.0mm.

I have double checked the parts listing... exhaust tube and carb/jets are the same. As are the carb tuning and ign settings. Max power is also developed at same revs (5000rpm).

So the Yamaha 9.9HP may be the easiest and cheapest make to convert to 15HP.

Reed image is those from my 15HP.... and one of the OB to ID its model.
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Old 20 April 2020, 13:01   #6
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MMSI: "Mmmmm SI" she said!
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Thanks SamT and Fenlander, just the sort of thing I need.

I missed out on a 15HP Yammie recently when I first knew about this project.

Nasher.
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Old 20 April 2020, 13:17   #7
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Can turn any 2 stroke Tohatsu's 6-8 HP and rebadged ones to a 9.8 as all 3 share same 178 CC powerheads, will need to install a 9.8 carb along increasing time advance some deg more.

For that matter, can convert a 2 strokes Tohatu's 9.9 to 15 HP and rebadged ones for peanuts costs, will need to install a new 15 HP throttle cam along increasing time advance some deg more. For both conversions will need to check the Factory Service Manual timing specs for each conversion.

Happy Boating
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