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Old 09-08-2016, 12:56 PM   #1
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Yamaha 9.9 HP 2 stroke to 15 HP

I picked up a 2011 / 2012 Yamaha 9.9 HP 2 stroke for our dinghy with idea of ( as per rumors ) converting to a 15 HP. The more I read, the more conflicting the stories are about the conversion. Has anyone done it ? How ? and what's involved ?

Thanks in advance, Frank B.
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Old 09-08-2016, 01:08 PM   #2
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I haven't done it, but if its possible to do, it should be easy enough to look in the parts books and see what's different.

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Old 09-08-2016, 01:11 PM   #3
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Quote:
Originally Posted by Ex Sailor View Post
I picked up a 2011 / 2012 Yamaha 9.9 HP 2 stroke for our dinghy with idea of ( as per rumors ) converting to a 15 HP. The more I read, the more conflicting the stories are about the conversion. Has anyone done it ? How ? and what's involved ?

Thanks in advance, Frank B.
I've never converted but have the 15hp Yamy and have been told the only difference is the carborator.
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Old 09-08-2016, 01:14 PM   #4
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Check to see if the engine blocks are both the same CC displacement. On some the reduced output is just a restrictor plate behind the carb. But if no plate, it might be cylinder porting and carb changes that would require replacing the block.

Lots of different ratings share the same blocks. In those cases, if a 9.9, it is a strong 9.9, and if a 15, it is a weak 15.

Especially trying to plane a dink, displacement means torque, and torque is what gets it up.
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Old 09-08-2016, 01:30 PM   #5
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The Yamaha 9.9 came in two models, one is standard the other is termed 'High Thrust'. The difference is in the lower unit where a low gear unit is applied to the high thrust. The conversion allows the 9.9 to perform at the 15 hp standard lower unit in performance as a pulling or pushing application, not a high speed performance. If you turn up the high thrust 9.9 you will not obtain any speed.
We have the high thrust 9.9 on our boat. It will push our 14,000# 27 foot S/D hull at 4.5 knots at 3/4 throttle and burn about a gallon per hour.
How to tell which you have? The standard lower unit uses a smaller diameter/pitch prop. the high thrust prop is large in both diameter/pitch. Very easy to tell the difference.
It serves as a great 'Get-A-Home' as well as a engine for salmon trolling'
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Old 09-08-2016, 01:43 PM   #6
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I'm w Ski on this.
Probably port timing and or carb bore.

But high performance ports just like valve timing have longer port/valve opening. The ports on the two stroke are taller and the stroke remaining after the piston covers the port is less. And low speed torque suffers just like a HP cam in a car.

But if the higher hp is achieved via carb bore alone the displacement (and bore and stroke) will remain the same. So looking up the displacement won't tell which engine it is.
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Old 09-08-2016, 01:46 PM   #7
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I have never done it but here is a link to the owners manual.....


http://www.yamaha-motor.com/assets/s...rrent_1794.pdf
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Old 09-08-2016, 01:49 PM   #8
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And here is the link on the parts required....


how do i convert my yamaha 9.9hp to 15hp - Page 2 - Yamaha Outboard Parts Forum
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Old 09-08-2016, 02:33 PM   #9
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Curious- The spec sheet provided by Smittyr77 has not reference to different reeds, or carb changes neither any reference to any stroke or bore change, There is a reference to the fact that not all the specs give are allocated to the specific model. Now, having said that
perhaps you boys will review the following. Simple to ask Google. Just saying as an owner of said engine first in the standard model which was converted to the high thrust model simply with the change to the lower unit. No other modifications.
Always happy to contribute. We all learn from sources on this forum.

High Thrust, Thrust Engine | Yamaha Outboards

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Old 09-08-2016, 03:24 PM   #10
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"Curious- The spec sheet provided by Smittyr77 has not reference to different reeds, or carb changes neither any reference to any stroke or bore change"


A cut and paste from the above link (post #8) gives you the information.....


"I have a 2010 9.9 hp 2 stroke bought it new ran it. For about 5 hours for break in and as others have said went to the yamaha parts online and compared all parts on the 9.9 to the 15 hp and the only mechanical part difference ( besides for the stickers )is the reed valves as someone mentioned the reed stoper alows the reed valves to open more on the 15 hp so not to mess with things I ordered the 15 hp reeds and *** what a difference, to compare the improvement , to go the same speed from the original reeds(9.9 hp) I had to open the throttle aprox 1/3 more, and top speed is better with the new reeds (15 hp) I can't remember the GPS speed difference as I did compare the two when I did this four years ago . Oh and there was not adjustments to carb ,timing, nothing just replace the bolt on reeds valves , the carb jets are the same for both engines "


"Just saying as an owner of said engine first in the standard model which was converted to the high thrust model simply with the change to the lower unit."


Change in lower units increases thrust at a higher cost but does nothing to increase Hp. Higher thrust can have value on heavier pushing loads at lower speeds but the OP was asking about increased HP.
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Old 09-08-2016, 07:38 PM   #11
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I modified my Yamaha 9.9 with good results. The 9.9 is just a detuned 15 HP. The only difference appears to be the reed valve. and the prop. You can buy the reed valve for about $50. The only difference that I could tell between the two valves is that the stops for the 9.9 were set closer, restricting the opening. I just carefully bent the stops to match the 15 HP version. I was told that there is also a restrictor plate behind the carburetor but mine didn't have one. Perhaps the original owner already removed it. I also changed the prop to the 15 HP version. I don't remember the part numbers.


I have a fairly heavy. 10 ft Achilles RIB. It will now plane with four adults aboard.
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Old 09-08-2016, 08:10 PM   #12
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Paul: Do you remember the prop you're running? We have a 15 hp and the props available are all over the place. Maybe we're running the wrong prop.
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Old 09-08-2016, 11:47 PM   #13
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[ the OP was asking about increased HP.[/QUOTE]
By golly you are correct, re read the op post. I will move on

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Old 09-09-2016, 12:49 AM   #14
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Quote:
Originally Posted by Paul Swanson View Post
I modified my Yamaha 9.9 with good results. The 9.9 is just a detuned 15 HP. The only difference appears to be the reed valve. and the prop. You can buy the reed valve for about $50. The only difference that I could tell between the two valves is that the stops for the 9.9 were set closer, restricting the opening. I just carefully bent the stops to match the 15 HP version. I was told that there is also a restrictor plate behind the carburetor but mine didn't have one. Perhaps the original owner already removed it. I also changed the prop to the 15 HP version. I don't remember the part numbers.


I have a fairly heavy. 10 ft Achilles RIB. It will now plane with four adults aboard.
Same experience here. I changed out the reed valve assembly (nothing else) and got significantly better performance. I can't quantify it, but top end is at least a few knots faster and, more importantly, the dinghy (also a 10 foot Achilles RIB) will reliably plane with 4 adults onboard. At 9.9 hp, getting on plane with 4 adults was marginal.

Here's the part I ordered: 63W-13610-00-00 YAMAHA REED VALVE ASY
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Old 09-09-2016, 05:47 PM   #15
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Al, the new prop is a 9-1/4 X10. I believe it is the standard prop for the 15 HP. It seems to work well although I have never actually measured the RPM at wot.

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Old 09-09-2016, 07:05 PM   #16
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I have the 15 two stroke, and have tried 10, 11 and 12 on my Avon 310 RIB. Settled on the 11 for general use. The 12 was fast with just me aboard - 22 knots - but struggled to get on a plane with two big people. The 10 could pop the boat on a plane easily but seemed underpropped when alone at WOT. For me the 11 is good, but if I were regularly carrying 3 or 4 people I'd probably use a 10. Great motor. Haven't measured WOT RPM.
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