# Mercury outboard: 9.9 to 15 hp?



## jcountry (Nov 29, 2010)

I have a 9.9 mercury 4-stroke, which is about a year old, and does a great job.... However, I have read that you can just change out to the 15 hp carb, and get all the extra power just from slapping a new carb on it. Supposedly, everything else on the 9.9 is the same as the 15.

-Anyone know if this would work?  Anyone here tried this?

-Thanks
-J


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## hipster dufus (Dec 2, 2010)

try the iboats forum. it is a tohatsu engine. i have a 2 yr old 9.8. i could be wrong, but i think the 15- 20 hp have the same engine head. the 8-9.8 share the same block, so i dont think u can.what do u have the engine on? mine is on a 1444 g3 mv jon. with 2 guys and gear i get9.5 mph, with 1 guy i can break 19 mph. i should have got the 20 hp with elec start. it is on sale now at online outboards.com, the camo edition is around 2700, with free shipping.a bargain. good luck


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## Twenty five ought six (Dec 2, 2010)

jcountry said:


> I have a 9.9 mercury 4-stroke, which is about a year old, and does a great job.... However, I have read that you can just change out to the 15 hp carb, and get all the extra power just from slapping a new carb on it. Supposedly, everything else on the 9.9 is the same as the 15.
> 
> -Anyone know if this would work?  Anyone here tried this?
> 
> ...



That's true, sort of, for 2 strokes, but I don't think that is the case for 4 strokes.

There is a lively debate about whether there is any real difference in 9.9 and 15 HP engines anyway.



hipster dufus said:


> try the iboats forum. it is a tohatsu engine. i have a 2 yr old 9.8. i could be wrong, but i think the 15- 20 hp have the same engine head. the 8-9.8 share the same block, so i dont think u can.what do u have the engine on? mine is on a 1444 g3 mv jon. with 2 guys and gear i get9.5 mph, with 1 guy i can break 19 mph. i should have got the 20 hp with elec start. it is on sale now at online outboards.com, the camo edition is around 2700, with free shipping.a bargain. good luck




Isn't it amazing how those Mercury decals are worth about $1000-1500?  

That black paint must be really expensive.


I think it is Tohatsu 2-strokes  that  have a cam stop that acts as a rev limiter, and by changing that or grinding it down, the max. revs can be increased, increasing HP.  Might search on that.


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## chadf (Dec 2, 2010)

Got a 14 hp I need to get rid of.
Just sitting around ........


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## seaweaver (Dec 2, 2010)

Twenty five ought six said:


> That's true, sort of, for 2 strokes, but I don't think that is the case for 4 strokes.
> 
> There is a lively debate about whether there is any real difference in 9.9 and 15 HP engines anyway.
> 
> ...



Try matching Mercury Phantom black! it cannot be done. There is no Blacker Black.
Sometimes there are exhaust porting differences between ratings.
cw


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## jcountry (Dec 2, 2010)

Interesting stuff...  I am pretty happy with the power I have on my jon now, but I mainly just wanted to see if this was for real, or just some kind of urban legend.

-Thanks for all the ideas!


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## Swamp Star (Dec 18, 2010)

chadf said:


> Got a 14 hp I need to get rid of.
> Just sitting around ........




Like brand new.


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## MD746 (Dec 19, 2010)

I know the 9.9 yamaha can be converted to 15hp. Not sure on the merc.


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## Twenty five ought six (Dec 19, 2010)

MD746 said:


> I know the 9.9 yamaha can be converted to 15hp. Not sure on the merc.



Are you sure about a four stroke?


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## Backlasher82 (Dec 19, 2010)

MD746 said:


> I know the 9.9 yamaha can be converted to 15hp. Not sure on the merc.



According to the specs, the Yamaha 9.9 is 212cc and the 15 is 362cc. It looks like the 15 and 20 share the same motor but the 9.9 is unique. 

http://www.yamaha-motor.com/outboard/products/subcatspecs/5/specs.aspx


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## MD746 (Dec 20, 2010)

Im sorry the Yamaha 2 stroke 9.9 with reed valve assembly changed to a 15 will get you 15hp.


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